The government says it wants a debate on the voting system at general elections - including a possible move to proportional representation.
The government says the current voting system works well
Justice Minister Michael Wills backed the current "first-past-the-post" system after the publication last week of a review of various alternatives.
But he told MPs he hoped it would stimulate debate on the issue.
Critics say the minister is keeping the door open for a deal with the Lib Dems in the event of a hung parliament.
Of the three main parties, only the Liberal Democrats are committed to proportional representation at Westminster, which it says is fairer than first past the post.
Labour has ruled out a move to PR in the past, after ditching its 1997 manifesto commitment to holding a referendum on the issue.
In an apparent softening of its line last week, it welcomed a review of voting systems in Scotland, Wales, Northern Ireland and European elections as a contribution to an ongoing debate on the issue.
But it rejected Lib Dem calls for a Commons debate and stressed that there would be no changes without reform of the House of Lords and a referendum.
Critics say the government is merely keeping its options open in case it has to form a pact with the Liberal Democrats.
Ken Richie, chief executive of the Electoral Reform Society, said: "If they find themselves at the time of the next election in a position in which they have to negotiate around the voting system they have a report that allows them to do that.
"On the other hand, they also have a report which allows them not to do anything, if they don't want to."
He urged the government to push ahead with a proper, structured debate on electoral reform - and what question should be asked in a referendum - rather than allowing change to be rushed through as a "concession to the Liberal Democrats".
Speaking earlier on Tuesday in the Commons, Labour MP Richard Burden questioned what form the electoral reform "debate" would take - and called for it to be thrown open to the public.
"Is there not now a case for asking voters themselves about what kind of electoral systems can best contribute to different forms of politics and what they want to see from our political system in this country?" asked Mr Burden.
Mr Wills replied that the review had been published to "inform the kind of debate" Mr Burden wanted to see.
He added: "These are important issues and we look forward to hearing the results of this debate."
The government's review of voting systems found that although PR had increased the number of parties represented in devolved assemblies it had confused some voters.
It also had a tendency to produce coalition governments and there was no guarantee it would increase turnouts, the report said.
Lib Dem Leader Nick Clegg last week described the review as a "huge anti-climax".
"Both the Labour and Conservative parties are happy to talk about getting people involved in politics but refuse to tackle the fundamental issue of electoral reform.
"That is why I have asked Gordon Brown and David Cameron to join me in a constitutional convention so we can begin to deliver the change that the British people deserve."