Jack Straw has said Tory MPs should have the "courage of their convictions" and back a referendum on ending the first past the post voting system.
On 9 February 2010, the justice secretary opened debate on plans for a referendum on switching to an alternative vote (AV) system where candidates are ranked in order of preference for election to Parliament.
Mr Straw denied it was a "cynical ploy" by Labour or part of a deal with the Lib Dems.
But the Tories say the £80m cost of a public vote is a waste of money.
Shadow justice secretary Dominic Grieve told Mr Straw: "I am truly sorry to see you being obliged to be associated with this guff."
He said the government had come up with the proposals "following the prime minister's belated conversion to the cause of electoral reform which he has so successfully and personally obstructed for over a decade".
He explained his support for the present, first-past-the-post system: "I happen to believe it delivers clear, clean results."
However, the two parties reached consensus on election night counts. Mr Straw revealed that the government would accept Tory amendments to oblige returning officers around the UK to start counts within four hours of the close of ballots wherever possible.
Liberal Democrat spokesman David Howarth said his party would support the government's bid for a referendum on AV, but only so that it could "radically" seek to amend it in favour of a plebiscite on a more proportional system.
But he added: "The question before us is not whether the proposal comes from a cynical or bad source. The question is whether the idea itself it worth supporting."