Furious MPs are demanding to know whether ministers plan to give murderers, rapists and paedophiles the vote as a result of a European court ruling.
The Government is expected to confirm formally on Wednesday that it will comply with a 2004 judgment of the European Court of Human Rights (ECHR) which ruled the UK's blanket ban on sentenced prisoners voting was illegal.
Political reform minister Mark Harper told MPs that a change in the law "isn't a choice; it's a legal obligation" and warned that any further delay could cost taxpayers millions of pounds.
But he strongly hinted that the Government will exclude the worst offenders from taking part in ballots.
Mr Harper was hauled before the House of Commons to explain the Government's intentions after reports that ministers were poised to end the 140-year-old ban, which applies to more than 70,000 people in British jails.
MPs challenged him to rule out the possibility of notorious criminals like Soham murderer Ian Huntley and triple police-killer Harry Roberts being given the vote.
Tory backbencher Gavin Williamson asked: "How is he going to make sure that rapists, murderers and paedophiles aren't going to have a vote?"
The minister said no decisions had yet been taken on exactly who would benefit from the change. But he stressed that the ECHR's ruling related to the blanket nature of the ban and promised MPs that the Government would "take into account" their concerns when framing new laws.
Reports have suggested that prisoners serving long sentences could continue to be excluded from the vote. There was some speculation that judges might be given the power to withhold it from individual offenders at the time of sentencing.
Mr Harper confirmed that Prime Minister David Cameron was "exasperated" at having to relax the UK's ban, but insisted the Government had no option but to comply with the ruling of the Strasbourg court.