CRITICS and supporters of Britain's electoral system are preparing to join battle over a bid to introduce proportional representation for general elections.

Scores of MPs are backing a new cross-party campaign seeking to scrap first-past-the-post voting, while the Conservatives are preparing to warn of the dangers of abolishing it.

MPs representing Labour, the Liberal Democrats, Scottish Nationalists, and Welsh separatist Plaid Cymru, and even some Tory activists, are backing the new organisation, which is billing itself as both cross-party and non-party.

The group, called Make Votes Count, hopes to rouse popular support for ditching first past the post and bringing in a new ''fairer'' voting system.

It will campaign for whatever alternative method is recommended later this year by the independent commission into the polling system headed by Lord Jenkins of Hillhead, the Liberal Democrat peer.

It has been tasked with finding an alternative which gives parties a ''broadly proportional'' share of seats compared to votes obtained, but also retains MPs' constituency links.

Charter 88, the constitutional pressure group, and the Electoral Reform Society have played a key role in setting up Make Votes Count. Celebrities and prominent figures are expected to be named among its supporters at its formal launch in Westminster on June 2.

On the same day, the Commons will debate a Tory motion lambasting PR. The main Opposition party intends to attack what it calls ''the dangers of changing Britain's successful and fair electoral system''.

Tory defenders of the existing system are planning to challenge MPs who want to dump Britain's traditional way of electing the Government. They will accuse Labour and the Liberal Democrats of backing PR to ensure they enjoy permanent coalition government at Westminster.

But Labour is split over the issue. Foreign Secretary Robin Cook is the party's best-known PR fan but a powerful group of senior Ministers, including Deputy Prime Minister John Prescott and Home Secretary Jack Straw, oppose a radical shake-up.

Tony Blair is said to be ''unpersuaded of the case for change''. The Labour Campaign for Electoral Reform, backed by over 100 Labour MPs, has thrown its weight behind Make Votes Count.

A campaign source said the group expected to attract especially strong support in Scotland because a form of PR, the additional member system, will be used next year to elect 56 of the 129 inaugural Members of the Scottish Parliament.

Dundee East Labour MP John McAllion is backing the group because ''while the current electoral system gives strong government, it has never given us fair government''.

He added: ''If we had had PR, we would never have had the Tories in Scotland.''

Ex-Scottish Office Minister Malcolm Chisholm said PR was ''part of the collaborative new politics that we're going to be pioneering in Scotland after next year''.