The war of words over electoral reform has flared up further as a senior Liberal Democrat branded first-past-the-post a "corrupt" system which had allowed Margaret Thatcher's Conservative government to practise "organised wickedness" in the 1980s.
Lib Dem president Tim Farron's comments, a week ahead of the May 5 referendum on the Alternative Vote, were the latest in a series of increasingly vicious attacks directed by the coalition partners at one another.
David Cameron and Nick Clegg shared the Government frontbenches in the Commons for the first time following an Easter break marked by open warfare between their parties on AV.
But there was little attempt to stage a show of coalition unity, with a stony-faced Mr Clegg shaking his head as the Prime Minister restated his opposition to change, then grinning broadly at Lib Dem jibes over Mr Cameron's election as Tory leader under a voting system with similarities to AV.
Meanwhile, Labour splits over the issue came to the fore, with the release of a dossier suggesting that more than half of the party's MPs intend to vote No next Thursday, despite their leader Ed Miliband's support for AV.
Many Labour opponents of AV - under which voters rank candidates in order of preference and votes are redistributed until one has more than 50% support - wore red badges in the House of Commons to show their support for the current system.
Former home secretary David Blunkett said that the Labour No campaign had the declared support of 130 of the party's 255 MPs and four in five of those councillors who have expressed a preference, as well as tens of thousands of activists.
"The Labour Party doesn't have a formal position but it's clear that the overwhelming majority of Labour people up and down the country are saying 'Vote Labour, vote No to AV'," said Mr Blunkett.
But another ex-home secretary Alan Johnson joined Mr Farron and the leaders of Ukip and the Greens at a press conference calling for a Yes vote.
Denouncing first-past-the-post as a "miserably disempowering system that belongs in the past", Mr Johnson said: "Labour should not become part of the establishment view. It is against our history, it is against our principles, it is against our ethos."