The government has no choice but to give some prisoners the vote because of a European court ruling, Constitutional Reform Minister Mark Harper has said.
The coalition "accepts... there is a need to change the law" and end the blanket ban on inmates being allowed to take part in ballots - which dates back to 1870 - Mr Harper told MPs, in response to an urgent question tabled by shadow justice secretary Sadiq Khan on 2 November 2010.
But he insisted no decision on which inmates would be allowed the vote had yet been taken.
The announcement follows consideration by the government and its Labour predecessor of a European Court of Human Rights judgment from October 2005, when Strasbourg ruled the outright ban was discriminatory in a case brought by prisoner John Hirst.
"This government accepts, as did the previous government, that as a result of the Strasbourg court in the Hirst case, there is a need to change the law.
"This isn't a choice, it's a legal obligation. Ministers are currently considering how to implement the judgment, and when the government has made a decision this House will be the first to know."
Mr Khan said the issue was a "matter of great concern to the public", saying that the ruling "opens the door to the possibility of serious offenders being given the vote".
He asked: "Can you explain how the government will ensure that serious offenders are not given the vote?"
Mr Harper responded: "No decisions have been taken and I am therefore not able at this time to answer any of your questions."