Prime Minister David Cameron accused Labour of "a descent into complete and utter opportunism" today after it emerged the party will vote against legislation paving the way for a referendum on voting reform for Westminster elections.
But shadow justice secretary Jack Straw insisted that Labour was trying to stop the Government using the voting reform legislation to smuggle in changes which would allow "gerrymandering" of parliamentary constituencies.
He challenged the Government to split the issues of voting reform and constituency boundaries into two separate Bills, promising Labour would then back the referendum.
Labour promised a referendum on switching from first-past-the-post to the Alternative Vote system in its general election manifesto.
But the shadow cabinet has now decided to oppose the Government Bill allowing it to be staged on May 5 2011, because of concerns over separate measures designed to produce parliamentary constituencies of uniform size across the country and to reduce the size of the House of Commons from 650 to 600 MPs.
Conservatives complain that the current boundaries require them to win more votes than Labour to gain the same number of MPs, because on average Tory seats have more constituents.
But Mr Straw insisted today the difference was only "marginal" and could be dealt with by the existing system of Boundary Commission reviews.
The changes proposed in the Parliamentary Voting System and Constituencies Bill would force through constituencies based on arithmetic calculations, without regard to historical boundaries and local loyalties, and would abandon the bipartisan commitment to local inquiries adjudicating on disputes, he claimed.
"If it had just been about the AV referendum, there would have been no difficulty in getting this Bill through," Mr Straw told BBC Radio 4's Today programme.
"What they have done is added to this Bill their very, very partisan proposals effectively for gerrymandering boundaries.
"We are not arguing about the equalisation of seats. We are arguing about the unnecessary reduction in the size of the House of Commons and we are particularly arguing about the way in which they are proposing to go about redrawing the boundaries.
"This hasn't come out yet but it will cause a huge fuss in every local area because they are abolishing and abandoning a bipartisan arrangement for local inquiries, chaired by judicially-qualified people, and going for a situation where the Boundary Commission treats the UK as four separate entities... and no other boundaries are required to be observed.
"Then they bash through, on an American system, an arithmetical formula. What that produces - as happens in the US - is not equalisation but the worst kind of gerrymandering in the world."
The Government could have Labour support for the AV referendum legislation "tomorrow" if they split it off from the measures to reshape constituencies, he said.
But Mr Cameron accused Labour of "a descent into complete and utter opportunism" over the issue.
The Prime Minister told Today: "They are the one party who in their manifesto had a commitment to the AV referendum and they are now backtracking on that.
"I know what it is like in opposition. I did almost five years as leader of the opposition. The temptation to jump on the bandwagon and be opportunistic is always there and it should always be resisted."
Mr Cameron brushed off the significance of unrest on the Tory backbenches over the timing of the referendum, which coincides with elections to the Scottish Parliament, Welsh Assembly and local councils in Northern Ireland and parts of England.
Some 43 Conservatives - along with two Labour MPs - have signed a motion warning that holding the votes concurrently risks clouding the issues and distorting the result by artificially inflating turn-out in areas of the country where other polls are taking place.
The growing row over voting reform sets the stage for a major test of the coalition, with Labour MPs lining up alongside rebel Tories to derail the proposals.
If the referendum does not make it through Parliament, disaffection among Lib Dem rank-and-file over the coalition agreement is likely to grow - especially as the party's support appears to have slumped since it was struck.
But Mr Cameron said he was "confident" of getting the legislation through.
He told Today: "Obviously I understand and I share the views of all Conservatives who, by and large, don't want to see the AV system. We prefer the current system.
"But we entered a coalition where, in return for having a referendum on AV, we will have - which I think we need in this country - equally-sized constituencies across the country and a smaller House of Commons."
He added: "I am very confident we will hold the referendum. It was part of the agreement between us and the Liberal Democrats, and it is very important to them."Reuse content