Britain will go to the polls on 5 May 2011 to decide whether we should change the way we vote.

The referendum was an important concession wrung from the Conservatives by their Liberal Democrat partners, and while it remains uncertain precisely how the two parties will align themselves for the campaign on the matter, the setting of the date is an important milestone in our electoral history.

The Labour leadership candidate David Miliband has strongly backed the reform, singling out the Alternative Vote (AV) system as his preferred option. On the Today programme this morning he said:

I think that it's important that we move to a system where every member of parliament has at least 50 per cent of the vote of their constituents.

Miliband himself received just over 52 per cent of the vote in his South Shields constituency, but a sizeable majority of his parliamentary colleagues were not so lucky. According to figures from the Electoral Reform Society, 434 MPs received less than 50 per cent of the vote -- that's 434 MPs who would be relying on redistributed preferences for a mandate under AV.

Some of the big names in this group include Ed Balls, Hazel Blears, Jon Cruddas, Ben Bradshaw, Danny Alexander, Oliver Letwin and David Davis.

The Conservatives have 179 MPs with less than 50 per cent of the vote but Labour has 181, or just over 70 per cent of the Parliamentary Labour Party. This last figure perhaps demonstrates why Labour is not whole-heartedly in favour of the referendum -- lots of previously safe seats would become very hard to predict under a new system.

Miliband's fellow leadership candidate Andy Burnham is not backing the reform, and is quoted in the Guardian as saying: "It is not my party's job to prop up the Liberal Democrats by helping them win a referendum that is important to them."

Had the most recent election been conducted using AV, the Labour Party would actually have has the smallest shift in its numbers, moving from 258 MPs to 262. But the Tories would have been even further from a majority, at 281, while the Liberal Democrats would have increased their share to 79.

A combined Lib-Lab government would thus have commanded a comfortable majority of 341 and kept the Tories out of Downing Street.

Full details of predictions under different voting systems are available from the Guardian's data blog.

But perhaps the most arresting aspect of the statistics produced by the Electoral Reform Society is the trend over time. In 1979, 32.4 per cent of MPs commanded less than 50 per cent of the vote, a number that has now more than doubled to 66.77 per cent.

With the 2010 election producing the highest ever number of MPs with less than 50 per cent of their local vote, the case has never been stronger for ditching the first-past-the-post system and restoring the link between constituency and MP.