The 'yes' campaign for the alternative vote hosted a high-profile event of cross-party MPs today.
Labour leader Ed Miliband, Green party leader Caroline Lucas and the former head of the Liberal Democrats Charles Kennedy all spoke at the event, extolling the benefits of a change to the voting system.
Deputy prime minister Nick Clegg was notably absent after Mr Miliband refused to share a platform with him.
The alternative vote allows voters to rank candidates for office by preference. The candidate with the lowest number of first preference votes is eliminated after each round of voting and their votes are then reassigned until one person passes the 50% mark.
'Yes' to AV argue this will make representatives more accountable, but the 'no' campaign says it undermines the principle of one person, one vote.