Gordon Brown is cutting it fine. The government's proposal for the alternative vote system of electing the Commons, subject to a referendum to be held by the autumn of 2011, is so late in this parliament that no one will put serious money on its chances of getting through. We have fewer than 90 days to the general election widely presumed to be held on 6 May, and only two-thirds of those days are available to pass legislation.

Yet the proposal is literally historic. Throughout the 19th century to early in the 20th, political reforms came thick and fast: the end of rotten boroughs, the gradual extension of the franchise, votes for all men, votes for older women and finally equal votes for men and women. And then we stopped. There has been less change in our electoral system for the Commons in the last 80 years than in any previous comparable period.

The alternative vote (AV) is a small step in the right direction, but is the most minimal change a Labour government could devise. AV will end "tactical" voting, whereby people vote for their second preference party to block the party they dislike most. It will allow everyone to vote for who they want, secure in the knowledge that if their preferred candidate has no chance and is eliminated in the count, their second and third preferences will be used for someone else. The process of elimination and counting goes on until someone has at last half of the votes in the constituency.

AV is therefore better because voters have more choice, and can honestly support who they want. Supporters of small parties know that their vote will no longer be wasted. It may have a role in helping to revitalise politics as a result.

But it is very similar to first-past-the-post in two key respects. Because it is based on single constituencies – a virtue for its proponents, who say they prize the constituency link – the parties continue to select one candidate each, and the voters only have one choice for each party.

That means that in the majority of parliamentary seats, the important decision about who should be the MP will continue to be taken in party caucuses rather than at the public ballot box. Although most MPs will have to reach beyond their tribal base to get the second preferences of other parties – an important discipline on unpleasant characters and behaviour – the choice for the voter remains very limited.

Compare AV in this respect with the Liberal Democrats' preferred option, the single transferable vote (STV), which is the system used in the Republic of Ireland, Scottish local government and in most Northern Irish elections. Each multi-member constituency has three to five members of parliament, so that each party has the incentive to put up two or more candidates. The voter therefore has the choice not only of party, but also of person.

Such a system is clearly the most liberal: it gives the maximum opportunity to the voter to express their preferences, and reserves the minimum power to the party machines. It is perfectly adapted for the world of the MPs' expenses scandal. Unlike AV, voters can stick with their party and vote for a "clean" MP or for an MP who shares their particular enthusiasms. With AV and first-past-the-post, voters have to change party to punish an individual MP.

In the Republic of Ireland, a third of all MPs who lose their seats in the Dail lose them to members of their own party, not partisan opponents. The system provides more discipline for MPs, and keeps them on their toes. MPs elected under STV have to provide a sterling service to their constituents, or they will find someone from their own party proving more attractive. The voter wrests power from the party.

Under AV, as under first-past-the-post, there would continue to be safe seats where the MP will effectively have a job for life. A third of all the constituencies in the country have not changed party since the second world war, and in those areas the MPs know that they can do pretty much anything without retribution. Research has shown that the worst expenses abuses occurred in safe seats where MPs face no threat of sacking by the electorate.

Conservative opposition to electoral reform gives the lie to David Cameron's pretence that he wants real change, and Labour's half-hearted commitment to the alternative vote is just a deathbed conversion from a party facing a historic defeat at the ballot box. Not only does AV fail to give voters the power they should have, but it also fails to remedy the unfairness of the present system.

Labour won 55% of the seats in the Commons with just 35% of the vote, and barely more than a fifth of the entire electorate. Yet AV can be even more disproportional when there are big swings from one side to the other such as in 1997 or 1979: under AV both Tony Blair and Margaret Thatcher would have had bigger majorities. The electoral system would continue to be like an ill-fitting corset attempting to squeeze all the diverse strands of opinion in our society into an inappropriate and deeply uncomfortable shape.

Instead, we need a parliament that properly represents our country in all its argumentative glory. Only when the Commons becomes an honest reflection of our people – not a fairground distorting mirror – will we be able to resolve the tensions and conflicts in our society. That is why the Liberal Democrats will move its proposal for the single transferable vote tomorrow.

Every voter, whether in an Essex marginal or a Welsh safe seat, should have a vote that counts, and every radical should want a political system that is fair. As our forebears knew, and we seem to have forgotten, you cannot build a fair society on an unfair political system. It is time for a new Reform Act.