Gordon Brown's government was last night accused of "fiddling with the voting system for partisan advantage" after reports suggested the Prime Minister and his colleagues are looking seriously at the adoption of a different electoral system for Westminster.

The Ministry of Justice, headed by Jack Straw, is to publish a draft Constitutional Reform Bill today, which, among other things, will include proposals for reforming the House of Lords. Ministers are said to be fearful that by modernising the upper chamber to create a Senate of around 400 paid representatives elected under a proportional representation (PR) system, they might give it greater legitimacy, which could be regarded by some as a challenge to the authority of the Commons.

Thus, Whitehall is due to begin - after May's local elections - a consultation, which, it hopes, will lead to greater voter engagement and turnout.

Among issues being considered are compulsory voting, opening polling stations at weekends and using the Alternative Vote (AV) system instead of the traditional first-past-the post (FPTP).

Under the AV system, voters do not mark an "x" next to a candidate's name but number the candidates in order of preference - depending on the number of would-be MPs. The candidate with the lowest number of votes is eliminated and his or her votes are reallocated according to voters' second preferences. This process continues until one candidate has more than 50%.

While this system retains the constituency link and provides the winning candidate with a technical majority, it also means that a seat will probably be decided upon by the second preferences of voters who supported the least popular candidate.

Mr Straw, the Justice Secretary and a known opponent of PR, is said to have warmed to AV, particularly as it keeps the constituency link. Significantly, perhaps, Nick Clegg's Liberal Democrats, ardent advocates of PR, have been told about Labour's proposals and might be prepared to accept AV if it were promoted as a staging post to their preferred goal.

However, some critics might sense deja vu. Recently, it was suggested in a Lords debate that the only reason Labour in 1997 offered Scottish devolution using PR was as a sweetener to the LibDems should there have been a hung parliament at Westminster.

Research has shown that because AV is a more proportionate system than FPTP, it could damage the Conservatives. Equally, however, the research also showed the Tories could gain an overall majority under AV with a smaller share of the vote than FPTP.

Nonetheless, Nick Herbert, for the Opposition, said: "It's a sure sign Labour fear they will lose the election when they now want to move the electoral goal posts to rig the result.

"Rather than fiddling with the voting system for partisan advantage, ministers should focus on rebuilding trust in politics, returning power to people and communities and tackling electoral fraud which is undermining the integrity of British democracy."

Last night, a Ministry of Justice spokeswoman told The Herald: "The government announced a significant programme of constitutional reform last July and plans to publish a draft constitutional renewal bill in due course.

"There are no current plans for specific papers on compulsory voting or the Alternative Vote system."