Any change will not affect the upcoming election
MPs are to be asked to vote next week on holding a referendum on electoral reform, the BBC has learnt.
Gordon Brown is backing a vote as the first step towards replacing the "first past the post" system of electing MPs with an "alternative vote" system.
Under an AV system, voters are asked to rank candidates by preference rather than putting a cross by a single name.
The BBC's Political Editor Nick Robinson said any change would not take effect before this year's election.
The Conservatives accused the prime minister of wanting to "fiddle" the voting system while the Lib Dems, who back a fully proportional electoral system, said the proposal did not go far enough.
The prime minister is set to announce on Tuesday that a referendum on changing the longstanding Westminster voting system could be held by the autumn of 2011 if the plan is approved by MPs.
The proposal follows a protracted debate with senior Labour ranks about the merits of a move to a different system.
WHAT IS ALTERNATIVE VOTE?
Voters rank candidates in order of preference and anyone getting more than 50% in the first round is elected.
If that doesn't happen, the candidate with the fewest votes is eliminated and their second choices allocated to the remaining candidates.
This process continues until a winner emerges
Under the AV system backed by Mr Brown - and currently used in Australia - voters rank candidates by preference on the ballot paper.
If no candidate wins more than 50% of votes in the first round, the votes of the candidates with the least support are, in turn, redistributed until a candidate gets a majority of votes cast.
Its backers say the system, while not a form of proportional representation, is fairer than the current system - in which many MPs are elected on a minority of the overall vote - while retaining the link between MPs and constituencies.
The expenses scandal has fuelled calls for wider political reform but there are several major obstacles standing in the way of any change.
Mr Brown wants the referendum plan to be inserted as an amendment into the Constitutional Reform and Governance Bill currently before Parliament but there may not be enough time for it to be approved before the next election.
Many Labour MPs remain sceptical, fearing it will damage their electoral prospects.
But our correspondent said a growing number of senior ministers were keen on the move as a way of trying to depict the Conservatives as opponents of political reform.
The Conservatives are strongly opposed to replacing the current "first past the post" system, saying it results in stable governments.
"Gordon Brown avoided a leadership election, bottled a general election and now wants to fiddle the electoral system," said Conservative chairman Eric Pickles
"It is not the voting system that needs changing; it's this weak and divided government."
The Liberal Democrats say the AV option is "a small step in the right direction" but not a substitute for a fully-fledged proportional system.
"If they agree it, this is a death-bed conversion from a party facing defeat at the general election," said its home affairs spokesman Chris Huhne, confirming the party would put forward its own amendment.
"It [the AV system] does not give voters real power over both the party and the person elected as MP."
Electoral reform campaigners say the public should get a say on different voting systems in any referendum.