Labour's proposed reform to the voting system for the House of Commons, the so-called Alternative Vote, would, in current circumstances, prove just as ineffective at reflecting the order of the parties as the existing system.

Our latest poll puts the Conservatives first on 32 per cent, just ahead of the Liberal Democrats on 31 per cent, while Labour are behind in third place with 28 per cent.

However, under the current system such a result would probably mean Labour would come first in seats, with a tally of 268, the Conservatives second, with 238, while the Liberal Democrats would clearly be third with just 112. The Liberal Democrats would lose out so badly because their votes are geographically too evenly spread. As a result they would secure a lot of second places, but fewer first places.

Labour do so well because Labour-held seats on average contain smaller electorates, a feature compounded by the fact that turnout tends to be lower in the party's strongholds. At the same time Labour's vote is more efficiently spread than that of the Conservatives, enabling it to pick up more seats by smaller margins.

Under the Alternative Vote the Liberal Democrats would fare much better. Under this system voters place candidates in rank order – 1, 2, 3 and so on. If no candidate wins 50 per cent of the first-preference vote, then the votes of candidates at the bottom of the poll are redistributed in accordance with those voters' second preferences, and so on, until someone passes the 50 per cent mark.

Nick Clegg's party tends to be everyone's second choice. As many as 68 per cent of Labour supporters say they would give a second-preference vote to the Liberal Democrats, as do 41 per cent of Conservatives.

So if the Liberal Democrats are running second in a constituency, they can hope to leapfrog into first place on the back of second preferences cast by third-placed Conservative or Labour supporters.

The party might win up to twice as many seats – 217 – as they would under the current system.

Yet it would be Labour, with 238 seats, that would still be the largest party. The Conservatives, meanwhile, would be a poor third with just 163 seats.

The Conservatives would lose out so badly because third-placed Labour voters are particularly ready to give a second preference to the Liberal Democrats. And the system would do nothing to correct the biases in the existing system that hurt the Tories.

Labour, though, would not be much helped by the system to leapfrog past the Conservatives in some seats. Almost as many Liberal Democrats (28 per cent) would give their second preference to the Conservatives as would to Labour (35 per cent), in sharp contrast to the position at the time of the last three elections.

So, under Labour's proposed alternative, not only would the party that was third in votes still come first in seats, but in addition the party that came first in votes would be a poor third in seats. One wonders whether voters will regard this as an improvement.

John Curtice is professor of politics at the University of Strathclyde. He is analysing the election opinion polls for 'The Independent'