Conservative MPs look set to be given a free vote on a Commons motion opposing votes for prisoners, it emerged today.
The motion, tabled by Tory former shadow home secretary David Davis and Labour's former Justice Secretary Jack Straw, is due for debate next Thursday and is expected to receive support from MPs of all parties.
If he attempts to impose a whip on Tory backbenchers, David Cameron may face his biggest rebellion since becoming Prime Minister.
Asked today whether the PM would instead allow backbenchers to vote according to their consciences, Mr Cameron's spokesman told reporters: "That would certainly be consistent with what we have been saying about the House expressing a view and us listening to that view. We will take that into account when we think about what we do next."
The Government is currently proposing to allow the vote to all inmates serving less than four years, in response to a European Court of Human Rights ruling which could otherwise open up the floodgates to compensation claims totalling millions of pounds.
But the move - which Mr Cameron said made him feel "physically ill" - has been met by stiff opposition from some MPs, and there have been indications that the vote may be restricted to those serving a year or less.
The motion tabled by Mr Davis and Mr Straw states that the decision on prisoners' votes should be one for democratically elected lawmakers and states that "no sentenced prisoner" should be granted the vote except those jailed for debt default or contempt of court.
The result of next Thursday's division will not tie the Government's hands.
But victory for the Davis-Straw motion would put enormous pressure on ministers to water down their proposals, which are expected to be put to the Commons before the summer recess.
The Government insisted it would extend voting rights only to general elections and those for the European Parliament despite warnings from a senior lawyer that Scottish and Welsh devolved polls and May's voting reform referendum could also be covered by the ruling.
Prisoner rights expert barrister Aidan O'Neill QC told MPs the Government was open to legal challenge from convicts if it did not allow them to participate in May 5 votes for the devolved administrations.
The referendum on switching from first-past-the-post to the alternative vote (AV) system for Westminster elections "may well be" covered as well, he told the Commons political and constitutional reform committee.
In response, a Cabinet Office spokeswoman said: "The Government has proposed that the right to vote will be restricted to UK Westminster Parliamentary and European Parliament elections only as that is the minimum currently required by law."
She added: "Removing the blanket ban on prisoners voting is not a choice, but a legal obligation as a result of a court ruling.
"Failure to implement the ruling would not only put the Government in breach of its international obligations but risk paying out taxpayers' money in compensation claims.
"The Government will do the absolute minimum to meet its obligations and will ensure that the most serious offenders are barred from voting."