Jack Straw accused the Tories of planning to "butcher" constituencies
Tory plans to cut the number of MPs are a "dangerous, destructive and anti-democratic" attempt to boost the party's influence, Jack Straw has said.
In a lecture on electoral reform, the justice secretary said a proposal to reduce the number of Commons seats by 10% was "outright gerrymandering".
David Cameron has said the move is needed to reduce the cost of politics.
But Mr Straw said Mr Cameron wanted to adjust constituency boundaries to make it easier for his party to win seats.
At present, variation in constituency sizes - sometimes by tens of thousands of voters - mean the Tories need a much higher share of the popular vote than Labour in order to win as many seats at a general election.
In a Hansard Society lecture on Tuesday evening, Mr Straw said: "The apparently virtuous call to cut the cost of politics is actually camouflage for a dangerous, destructive and anti-democratic piece of gerrymandering.
"The Conservative Party proposes to cut the number of MPs by 10% without testing the will of the people in a referendum and without any proper effort to seek any kind of cross-party consensus.
"That is because their proposal is not about cutting the cost of politics, as they claim. It is about advantaging the Conservative Party."
Mr Straw also warned that having fewer MPs would result in a poorer deal for constituents.
He said it would detach politicians from their constituencies and add considerably to the workload of remaining MPs.
"In stark contrast to Labour's agenda for moving towards a new politics on the basis of popular consent, the Conservatives aim to butcher scores of constituencies for sordid political ends," he added.
Shadow Justice Secretary Dominic Grieve said many MPs were already representing constituencies larger than under his party's proposals.
"To suggest that MPs would be unable to do the job is ludicrous," he said. "Under our plans MPs would share the workload equally."
The Liberal Democrats have also pledged to cut the number of MPs, ministers and government departments if they win the next election, which is widely expected on 6 May.