AT least half of the House of Commons’ 646 MPs will be swept away at the general election, as voters take revenge on the political classes for the expenses scandal.

The departure of 325 members of parliament as a result of forced resignations, retirement and defeat at the polls would represent the biggest clear-out of parliament since 1945.

As many as 30 will be forced to resign directly because of the expenses scandal, while whips expect more than 200 to quit because they are unable to cope with continued public anger. Up to 90 MPs will be voted out in the election.

Research conducted by The Sunday Times and Professor Colin Rallings, director of the elections centre at Plymouth University, suggests that about 170 Labour MPs will not defend their seats while 55 Conservatives are also expected to retire.

Dozens more MPs from all parties are likely to lose their seats as voters kick out incumbents, accused of profiting from their allowances.

Rallings said: “If, as the current polls suggest, the Conservatives were to win the general election with an overall majority of 80 seats, it is likely that fully half of MPs in the new House of Commons will be new, a product both of incumbents being defeated and MPs retiring. It would be without parallel since 1945.”

The disclosure comes as Gordon Brown, desperate to retake the political initiative, examines radical constitutional reforms including the introduction of four-year, fixed-term parliaments.

Supporters of the plan say that removing the power of a future prime minister to determine the date of the general election would create a fairer system. However, it would tie the hands of the Conservatives, should they win the election.

Brown is facing growing pressure from senior Labour figures to resolve uncertainty about the date of the general election. David Cameron, the Conservative leader, has called for an autumn election.

Charles Clarke, the former home secretary, said: “It would be best now for Gordon Brown to set the election day as the first Thursday in May 2010 and stick to that date.”

Andrew MacKay, Cameron’s former chief political adviser, yesterday became the biggest casualty of the expenses scandal when he announced he would step down at the next election, following an ultimatum from the Conservative leader. He said: “I believe I could be a distraction at a time when he is working to get elected as prime minister.”

The brother of his wife, the MP Julie Kirkbride, sometimes lived rent-free in their taxpayer-funded Bromsgrove home.

Ian McCartney, the former Labour party chairman, also announced his retirement in the wake of the scandal. McCartney, the MP for Makerfield, has paid back £15,000 of expenses after buying at the taxpayers’ expense an 18-piece dinner set, champagne flutes and wine glasses, a £700 dining table and chairs and sofas worth £1,328. McCartney, who has had heart surgery, said he was going because of health problems.

MacKay’s departure means six MPs have already been directly forced out by the expenses scandal. More resignations are likely this week as the Labour and Conservative “star chambers” meet to discuss the most serious abuses.

McCartney’s retirement brings to 38 the total number of Labour MPs who have announced their decision to go, for a range of reasons.

Senior whips believe the number could rise to 170 - half the parliamentary party. “We think that one out of every two MPs will ultimately decide to go,” said a whip. “Those likely to leave are split evenly between marginals and safe seats: 170 sounds like a huge figure and it is, but that reflects the level of despair in the party.”

Whips believe that there will be a flurry of resignations after the Whitsun recess, when MPs consult their frustrated families and angry constituents.

A total of 20 safe Tory seats will definitely become vacant at the next election, including those of grandees who have resigned after being accused of expenses abuses. Party whips estimate that a further 35 MPs, mainly over the age of 60, will shortly announce their retirement.

The Liberal Democrats expect about five departures and a similar number of MPs from minor parties will retire.

Rallings estimates that on current polling trends a further 60 MPs will leave parliament as a result of being defeated at the polls. He also estimates that a further 30 MPs will go as a result of “churn” - seats changing hands between the Lib Dems, nationalists and main parties. This takes the total number of MPs leaving the Commons to 325. That figure would rise further if Cameron’s majority proves to be larger than 80 or if independent antisleaze candidates such as Esther Rantzen, the television presenter, are elected to parliament.

In the 1997 Labour landslide election, just under 250 MPs left parliament through retirement or defeat at the polls. In the 1945 Labour landslide, half of the MPs returned to parliament were new. However, this was exceptional because of the second world war.

Harriet Harman, Labour’s deputy leader, and Jack Straw, the justice secretary, are drawing up sweeping reforms to modernise parliament in the wake of the expenses row.

A spokesman said they would “certainly” consider axing prime ministerial control over election timing.

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