Under electoral reform plans championed by Gordon Brown, the country would be asked to choose whether to switch to the alternative vote (AV) method.

The Prime Minister hailed the change as part of a "new politics" which could restore public trust in Westminster in the wake of last year's expenses scandal.

But the Tories will oppose the move which opposition leader David Cameron has dismissed as a "fiddle", accusing the PM of "a cynical attempt to save his own skin".

Amendments tabled by Justice Secretary Jack Straw to the Constitutional Reform and Governance Bill would require a referendum to be held by the end of October 2011.

It would offer a straight choice between the existing system and AV, under which voters would rank candidates in order of preference.

The Liberal Democrats will seek to change the question to offer the choice of a more proportional system - the single transferable vote - instead of AV.

They also want to bring forward the last date of the referendum to May next year, and close what they say is a loophole that would allow an incoming Conservative administration to abandon the vote without the need for fresh primary legislation.

Liberal Democrat justice spokesman David Howarth said: "Voters deserve a real choice - between the discredited status quo and a system where every vote matters and there are no safe seats.

"The Alternative Vote system is a very small step in the right direction, but it is not proportional and it does not give voters enough power over the party and the person elected as MP.

"It is also unacceptable that Labour's amendments would make it childishly easy for the next Government to kill a referendum without further legislation."

He added: "By acting purely out of naked self-interest, the Conservative Party has long been a roadblock to electoral reform."

Labour former minister Frank Field tabled a bid to put the choice of a "two-round run off" alongside AV and first-past-the-post on the referendum paper.

Campaigners for voting system change welcomed the AV proposal as a "small step" towards making the electoral system more representative.