The last supper was most definately the Passover meal. Anything fermented or leavened was strictly forbidden by the Mosaic law during the Passover. Below are some Bible verses and commentaries which show this to be true.
Exodus 12:14-20 KJV
So this day shall be to you a memorial; and you shall keep it as a feast to the LORD throughout your generations. You shall keep it as a feast by an everlasting ordinance. 15Seven days you shall eat unleavened bread. On the first day you shall remove leaven(7603) from your houses. For whoever eats leavened(2557) bread from the first day until the seventh day, that person shall be cut off from Israel. 16On the first day there shall be a holy convocation, and on the seventh day there shall be a holy convocation for you. No manner of work shall be done on them; but that which everyone must eatthat only may be prepared by you. 17So you shall observe the Feast of Unleavened Bread, for on this same day I will have brought your armies out of the land of Egypt. Therefore you shall observe this day throughout your generations as an everlasting ordinance. 18In the first month, on the fourteenth day of the month at evening, you shall eat unleavened bread, until the twenty-first day of the month at evening. 19For seven days no leaven(7603) shall be found in your houses, since whoever eats what is leavened(2557), that same person shall be cut off from the congregation of Israel, whether he is a stranger or a native of the land. 20You shall eat nothing leavened(2557); in all your dwellings you shall eat unleavened bread.
Strongs Concordance
H2557 chamets khaw-mates'
From H2556; ferment, (figuratively) extortion:leaven, leavened (bread).
H7603 se'or seh-ore'
From H7604; barm or yeast cake (as swelling by fermentation):leaven.
H2556 chamets khaw-mates'
A primitive root; to be pungent; that is, in taste (sour, that is, literally fermented, or figuratively harsh), in color (dazzling):cruel (man), dyed, be grieved, leavened.
Spiros Zodhiates Th.D.
2557 This masc. noun is derived from chamets(2556). It is anything which is fermented or leavened (even ill-gotten wealth). The normal process of bread-making included some form of yeast to make the bread rise i.e., the natural fermentation of benign bacteria. During the Passover, no one was allowed to eat leavened bread (Ex. 12:15; 13:3,7). Unleavened bread signified the suddenness of the deliverance of the Israelites by God (Deut. 16:3). Ordinary leavened bread was prohibited on other occasions (Ex 23:18; 34:25; Lev. 2:11; 6:17), but allowable under certain circumstances (Lev. 7:13; 23:17; Amos 4:5). It later became a symbol of corruption and impurity in the Jewish religious system (see Matt. 16:12; Mark 8:15; 1 Cor. 5:8).
2556 to be sharp, be sour, be salted, to be leavened; dazzling (in color); to be violent; to be excited, be bitterly moved (Ps. 73:21). The basic meaning of the root of these forms was fermentation (Ex. 12:19, 20, 34, 39)
Youngs Literal Translation of the Bible Ex. 12:14-20
And this day hath become to you a memorial, and ye have kept it a feast to Jehovah to your generations;a statute age-during; ye keep it a feast. 15Seven days ye eat unleavened things; onlyin the first day ye cause leaven to cease out of your houses; for any one eating anything fermented from the first day till the seventh day, even that person hath been cut off from Israel.
16And in the first day is a holy convocation, and in the seventh day ye have a holy convocation; any work is not done in them, only that which is eaten by any personit alone is done by you, 17and ye have observed the unleavened things, for in this self-same day I have brought out your hosts from the land of Egypt, and ye have observed this day to your generationsa statute age-during.
18In the first month, in the fourteenth day of the month, in the evening, ye do eat unleavened things until the one and twentieth day of the month, at evening; 19seven days leaven is not found in your houses, for any one eating anything fermentedthat person hath been cut off from the company of Israel, among the sojourners or among the natives of the land; 20anything fermented ye do not eat, in all your dwellings ye do eat unleavened things.
Hastings Dictionary of the Bible
The leaven both of OT and of NT may be assumed to have always consisted of a piece of fermented dough from a previous baking. There is no clear trace, even in the Mishna, of other sorts of leaven, such as the lees of wine or those enumerated by Pliny (Hist. Nat. xviii. 26).
A Dictionary of the Bible by William Smith, LL.D.
Various substances were known to have fermenting qualities; but the ordinary leaven consisted of a lump of old dough in a high state of fermentation, which was mixed into the mass of dough prepared for baking. The use of leaven was strictly forbidden in all offerings made to the Lord by fire. During the Passover the Jews were commanded to put every particle of leaven from the house. The most prominent idea associated with leaven is connected with the corruption which it had undergone, and which it communicated to bread in the process of fermentation. It is to this property of leaven that our Saviour points when He speaks of the leaven (i.e. the corrupt doctrine) of the Pharisees and of the Sadducees, Matt. 16:6; and St. Paul, when he speaks of the old leaven. 1 Cor. 5:7
Vines Expository Dictionary of Old and New Testament Words
Leaven, sour dough, in a high state of fermentation, was used in general in making bread. It required time to fulfill the process. Hence, when food was required at short notice, unleavened cakes were used, e.g., Gen. 18:6; 19:3; Ex. 12:8. The Israelites were forbidden to use leaven for seven days at the time of the Passover, that they might be reminded that the Lord brought them out of Egypt in haste, Deut. 16:3, with Ex. 12:11; the unleavened bread, insipid in taste, reminding them, too, of their afflictions, and of the need of self-judgement, is called the bread of affliction., leaven was utterly inconsistent in offerings which typified the propitiatory sacrifice of Christ.
The Companion Bible by Kregel Publications-Appendix 38
Its first occurance in Ex. 12:15 significantly marks it as something to be put away. There is no dispute as to the meaning of the word, which is sour or fermenting dough.
Eastons Bible Dictionary
(1.) Heb. seor (Ex. 12:15, 19; 13:7; Lev. 2:11), the remnant of dough from the preceding baking which had fermented and become acid.
(2.) Heb. hamets, properly ferment. In Num. 6:3, vinegar of wine is more correctly fermented wine. In Ex. 13:7, the proper rendering would be, Unfermented things [Heb. matstsoth] shall be consumed during the seven days; and there shall not be seen with thee fermented things [hamets], and there shall not be seen with thee leavened mass [seor] in all thy borders. The chemical definition of ferment or yeast is a substance in a state of putrefaction, the atoms of which are in a continual motion.
The use of leaven was strictly forbidden in all offerings made to the Lord by fire (Lev. 2:11; 7:12; 8:2; Num. 6:15). Its secretly penetrating and diffusive power is referred to in 1 Cor. 5:6. In this respect it is used to illustrate the growth of the kingdom of heaven both in the individual heart and in the world (Matt. 13:33). It is a figure also of corruptness and of perverseness of heart and life (Matt. 16:6, 11; Mark 8:15; 1 Cor. 5:7, 8).
Holman Bible Dictionary
LEAVEN (Leeaw' vehn) A small portion of fermented dough used to ferment other dough and often symbolizing a corruptive influence. The common bread of Old Testament times was made with leaven. Such bread was acceptable as wave offerings for the priests and as loaves to accompany the peace offerings (Lev. 7:11-13; 23:17). However, bread made with leaven or honey, both associated with the process of fermentation and thus a source of corruption, was never to be used as offerings to be burned on the alter (Lev. 2:11-12). Unleavened bread was also prepared in times of haste (1 Sam. 18:24) and was required for the Feast of Unleavened Bread which was celebrated in conjunction with the Passover festival (Lev. 23:4-8). This unleavened bread, or bread of affliction, reminded the Israelites of their hasty departure from Egypt and warned them against corruptive influences (Ex. 12:14-20).
Adam Clarkes Commentary on the Old Testament
Unleavened bread , from  , to squeeze or compress, because the bread prepared without leaven or yeast was generally compressed, sad or heavy, as we term it. The word here properly signifies unleavened cakes; the word for leaven in Hebrew is  , which simply signifies to ferment. It is supposed that leaven was forbidden on this and other occasions, that the bread being less agreeable to the taste, it might be emblematical of their bondage and bitter servitude, as this seems to have been one design of the bitter herbs which were commanded to be used on this occasion; but this certainly was not the sole design of the prohibition: leaven itself is a species of corruption, being produced by fermentation, which in such cases tends to putrefaction. In this very light St. Paul considers the subject in this place; hence, alluding to the passover as a type of Christ, he says: Purge out therefore the old leavenfor Christ our passover is sacrificed for us: therefore let us keep the feast, not with old leaven, neither with the leaven of malice and wickedness, but with the unleavened bread of sincerity and truth; 1 Corinthians 5:6-8.
Full Life Study Bible
USE OF WINE IN LORDS SUPPER. Did Jesus use fermented or unfermented grape drink when He instituted the Lords Supper (Mat. 26:26-29; Mark 14:22-25; Luke 22:17-20; 1 cor. 11:23-26)? The following data support the conclusion that what Jesus and His disciples drank was unfermented grape juice.
1) Neither Luke nor any other Biblical writer uses the word wine (Gk. oinos) in regard to the Lords Supper. The first three Gospel writers use fruit of the vine (Mat. 26:29; Mark 14:25; Luke 22:18). Unfermented wine is the only true natural fruit of the vine, containing approximately 20 percent sugar and no alcohol. Fermentation destroys much of the sugar and alters what the vine produced. Fermented wine is not the product of the vine.
2) The Lords Supper was instituted when Jesus and His disciples were eating the Passover. The Passover law in Ex. 12:14-20 prohibited, during Passover week, the presence and use of seor (Ex. 12:15), a word refrring to leaven, yeast, or any agent of fermentation. Seor in the ancient world was often obtained from the thick scum on top of fermenting wine. Furthermore, all hametz (i.e., anything containing any fermentation) was forbidden (Ex. 12:19; 13:7; see 13:7 , note). God had given these laws because fermentation symbolized corruption and sin (cf. Mat. 16:6, 12; 1 Cor. 5:7-8). Jesus, the Son of God, fulfilled the law in every requirement (Mat. 5:17). Thus, He would have followed Gods law for the Passover and not used fermented wine.
3) A rather lively debate has taken place over the centuries among Jewish rabbis and scholars as to whether fermented products of the vine were allowed in the Passover. Those who held to a stricter and more literal interpretation of the Hebrew Scriptures, especially Ex. 13:7, insisted that no fermented wine was to be used on this occasion.
4) Some Jewish sources affirm that the use of unfermented wine at the Passover was common in N.T. times. For example, According to the synoptic Gospels, it would appear that on the Thursday evening of the last week of His life Jesus with His disciples entered Jerusalem in order to eat the Passover meal with them in the sacred city; if so, the wafer and the wine of
the communion service then instituted by Him as a memorial would be the unleavened bread and the unfermented wine of the Seder service (see Jesus, The Jewish Encyclopedia, 1904 edition, v.165)
5) In the O.T. fermented drink was never to be used in the house of God, nor were the priests allowed to draw near to God in worship while drinking intoxicating beverages (see Lev. 10:9, note). Jesus Christ was Gods High Priest of the new covenant, drawing nigh to God for the sake of His people (Heb. 3:1; 5:1-10).
6) The value of a symbol is determined by its capacity to conceptualize the spiritual reality. Therefore, just as the bread represented the pure body of Christ and had to be unleavened (i.e., uncorrupted with fermentation), the fruit of the vine, representing the incorruptible blood of Christ, would have been best represented by juice that was unfermented (cf. 1 Pet. 1:18-19). Since Scripture states explicitly that the process of corruption was not allowed to work in either the body or blood of Christ (Ps. 16:10; Acts 2:27; 13:37), both of these are properly symbolized by that which is uncorrupted and unfermented.
7) Paul instructed the Corinthians to put away spiritual yeast, i.e., the fermenting agent of malice and wickedness, because Christ is our Passover (1 Cor. 5:6-8). It would be inconsistent with the goal and spiritual requirement of the Lords Suppper to use something which was a symbol of evil, i.e., something with leaven or yeast.
Exodus 13:7 Note During the week of the Passover, leaven (Heb. Seor, i.e., any yeastlike substance capable of producing fermentation in dough or a liquid) and any thing leavened (Heb. Hamets, i.e., anything that had undergone fermentation or anything with yeast in it) was to be removed from the homes of the Israelites (cf. 12:15,19). In 12:15 and 13:7 hamets is translated leavened bread; however, the literal meaning of the word is fermented thing.
Leviticus 10:9 Note Abstinence from intoxicating wine was required of all priests when performing their religious duties. (1)They were expected to be a holy vessel before God and the people whom they were to teach soberly the way of God (vv.10-11; see Eph. 5:18, notes). (2) The violation of this ordinance about abstinence was serious enough to incur the penalty of death. The point is clearGod considered any amount of intoxicating drink incompatible with His highest standard of godliness, wise discernment, and sensitivity to the leadership of the Holy Spirit (see Prov. 23:29-35; 1 Tim. 3:3, note; Titus 2:2, note).