Referendum: 'Opportunism' row as Labour opposes election reforms

But Shadow Justice Secretary Jack Straw said Labour was trying to stop the Government using the voting reform legislation to smuggle in changes to 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 a separate measure designed to produce parliamentary constituencies of uniform size and to reduce the number of MPs from 650 to 600.

Mr Straw said the changes proposed in the Parliamentary Voting System and Constituencies Bill would force through constituencies based on arithmetic, without regard to historical boundaries and local loyalties creating "the worst kind of gerrymandering in the world".

"If it had just been about the AV referendum, there would have been no difficulty in getting this Bill through," Mr Straw said

But Mr Cameron accused Labour of "a descent into complete and utter opportunism" over the issue.

The Prime Minister said: "They are the one party who in their manifesto had a commitment to the AV referendum and they are now backtracking on that. "

Mr Cameron brushed off the significance of unrest on the Tory backbenches, after 43 Conservatives signed a motion opposing linking the vote on he voting change to other polls on the same day.