The Opposition Day Debate in the Commons on "The dangers of changing Britain's successful and fair electoral system" also comes on the same day as the launch of the Yes campaign for a referendum to change the way we elect MPs to Wesminster.

Labour made a commitment to hold such a referendum in its 1997 manifesto.

Officially, the government remains persuadable on Proportional Representation with Cabinet members such as Robin Cook for it and John Prescott against it, while the Prime Minister himself has said he is not fully persuaded by the case for PR.

The Jenkins commission

The remit of the Jenkins commission, currently investigating the electoral system, was described by the Home Secretary, Jack Straw, as an exploration of any "appropriate system or combination of systems in recommending an alternative to the present system for parliamentary elections to be put before the people in the government's referendum."

Under a system of Proportional Representation, of which there are many variations, seats in Parliament are allotted according to the proportion of the total vote received by each party and not according to the votes received in a single seat or constituency.

Supporters says that this means every vote counts, while opponents say the system gives too much power to minor parties, which often end up as power brokers after an election as PR rarely produces an absolute majority for any party.

Many systems of PR also force a move away from MPs representing individual constituencies. Instead MPs might be selected from party lists, each party having a number of MPs proportional to their share of the vote.

Tory fears

PR's frequent reliance on coalition governments and its lack of constituency representation are two of the Conservatives' main quarrels with the system.

They also take issue with the terms of reference for the Jenkins Commission which the Conservative Party chairman, Lord Parkinson, called a "a rigged body with a rigged remit - with the sole purpose of coming up with a rigged voting system to keep Labour in power at any cost."

But as those who favour PR point out, the current "first-past-the-post" system which the Conservatives support also has its drawbacks.

Under this system a Labour government lost power in 1951 despite receiving more votes than the Conservatives - and the same thing happened to the Tories who lost out to Labour in 1974.

The Liberal Democrats are just as firm in their support for PR as the Tories are against it, while Labour has yet to make up its mind.