A referendum to change Britain's first-past-the-post voting system could be held on the same day as the general election next spring, under proposals being discussed by ministers.

The idea is gaining support in the Cabinet, and Labour now looks certain to fight the next election on a firm commitment to scrap the current voting system.

Although ministers have ruled out a switch to the fully proportional system sought by the Liberal Democrats, they plan to allow voters to list candidates in order of preference, with the bottom candidate dropping out until one contender enjoys more than 50 per cent support.

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This Australian-style alternative vote (AV) system is one leading option. Another is "AV-plus", recommended by an inquiry chaired by the late Lord Jenkins of Hillhead in 1998 but left on the shelf by the Blair government, which commissioned it. This would also see the election of "top-up" MPs in proportion to the votes cast.

The Government's Democratic Renewal Council, chaired by Gordon Brown, met this week. Although no final decision was taken, The Independent has learnt that the options it is considering include:

* Rushing through legislation before the election to allow a referendum on electoral reform shortly afterwards;

* A polling day referendum on the principle of changing the system, to be followed by a second plebiscite if there were a "yes" vote;

* A polling day referendum on a switch to AV or "AV plus", to be implemented at the following general election;

* A Labour manifesto commitment to change the system if the party retains power.

One cabinet source said: "The idea of a referendum on election day is on the agenda. It is a very live issue."

Yesterday, senior Labour figures denied that the move was a desperate attempt to cling on to power by muddying the waters at the election and pitching for the votes of people who support reform.

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"People have a way of getting the government they want," one Labour source said. "It could easily go the other way – voters might be more likely to vote against a proposal made by the government of the day."

The driving force for the last-minute push by Labour after 12 years in power is the expenses controversy. The party's internal polls show it remains a huge issue for the public.

"Two things matter on the doorsteps: the economy and expenses," one Labour adviser said last night. "The expenses row means people are not listening to us on the economy. So we have to try to restore trust, and one way is to change a discredited voting system."

The Tories strongly oppose electoral reform, and are bound to accuse Labour of "moving the goalposts" and trying to curry favour with the Liberal Democrats in case there is a hung parliament.

Tories point to a YouGov survey for the Electoral Reform Society which found that, among people intending to vote Liberal Democrat, 9 per cent say a referendum would make them "much more likely" to vote Labour and 21 per cent "somewhat more likely" to do so. Only 4 per cent would be deterred.

Even if the public voted for reform in an election-day referendum, a Cameron government would be unlikely to act on it. Similarly, if legislation were forced through calling for a post-election referendum, it could be overturned by an incoming Tory administration.

Labour officials say that raising the issue could allow the party to portray Mr Cameron as "anti-reform". There is frustration in Labour circles that the Tory leader has appeared to outflank the Government by responding quickly during the expenses saga.

Many Labour MPs oppose reform and grassroots activists who want to keep the present system are calling for a debate on the issue at the party's annual conference in Brighton this month.

Electoral reform: The options

The Alternative Vote (AV)

Instead of voting for one candidate, people can rank the candidates in order of preference. The candidate coming last drops out and second preferences are redistributed until one candidate enjoys more than 50 per cent. Retains link between MPs and their constituents. Used in Australia.

AV Plus

People would have two votes – one for constituency MP (as under AV) and one for a top-up list in 65 areas in England, eight in Scotland, four in Wales and two in Northern Ireland. Some 80 to 85 per cent of MPs would be elected in individual constituencies. The rest would be "top-up MPs" to reduce disproportionality and geographical divisions of first-past-the-post system.