Labour leader Ed Miliband praised the values of the alternative vote system at the campaign event:
"For most of the last 80 years, there has been one Conservative party - and others competing with Labour for progressive votes.
"The results, over the years, speak for themselves. No wonder the Tories back the current system.
"They know Britain is not a fundamentally Conservative country. But with first past the post, they too often govern when progressive forces are divided.
"This Tory-led government and its current alliance of power with the Liberal Democrats does not change my belief that there is a progressive majority in this country.
''The tragedy for progressive politics in Britain has been that division on the centre and left has handed a united right victory after victory.
"Britain deserves an electoral system that fairly reflects voters' views.
"If there is a Conservative majority, they deserve the same right. But rule without a mandate, simply because the majority is divided, is unfair on voters."