The debate is expected to show the deep divisions on Labour's side over the issue of voting reform.

Many senior backbenchers are due to use the opportunity to voice their opposition to proportional representation for Westminster - recommended last week by the Jenkins Commission.

Others in favour of reform will just as firmly press the case for change.

Home Secretary Jack Straw, who is strongly opposed to replacing the present first-past-the-post system, is to open the debate for the government.

Jack Straw is firmly anti-voting reform

Chaired by Liberal Democrat peer Lord Jenkins, the Independent Commission on the Voting System proposed a new "Alternative Vote Top-Up" electoral system.

Most MPs - up to 85% - would be elected on a constituency basis. The rest would be chosen from "top-up" lists, designed to iron out imbalances in how the parties are represented in parliament, compared with voters' overall preferences.

The Liberal Democrats have long campaigned for PR and welcomed the commission's report.

But with his party and cabinet is split on the issue, there are huge pressures on the prime minister to abandon any thought of reform before the next election.

Labour pledged before last year's election to hold a referendum on reform in this parliament, but ministers have been backing away from the promise in recent months.

For and against

The pro-reform camp includes Foreign Secretary Robin Cook, Culture Secretary Chris Smith, Northern Ireland Secretary Mo Mowlam and International Development Secretary Clare Short.

Backbencher Stephen Twigg heads up the Labour Campaign for Electoral Reform, which is leading the case for change within the party.

Lib Dem leader Paddy Ashdown: His party wants the referendum

They say reform would more accurately reflect the wishes of the electorate.

The anti-reform camp includes, as well as Mr Straw, Deputy Prime Minister John Prescott, Leader of the House Margaret Beckett and Education Secretary David Blunkett.

Anti-reformers, such as former Tory prime minister John Major, argue that any form of PR would lead to coalition governments unable to take decisions, and would give unfair power to minority parties.