Labour's Shadow Chancellor Ed Balls has said the party will back David Cameron if he chooses to fight an ultimatum issued by human rights judges giving Britain six months to change the law on prisoners' voting rights.
Mr Balls said there was cross-party agreement about not giving the vote to prisoners and the ruling by the European Court of Human Rights (ECHR) was the "wrong thing".
"This is one of those times in politics where there is cross-party consensus," he told ITV Daybreak.
"The court first said this in 2004, that prisoners should be able to vote, and Labour then said we disagree and we did not implement it.
"I am all in favour of prisoners having the right kind of support and being rehabilitated but voting is one of the things I think you give up if you go to prison.
"So we all agree that this is the wrong thing."
The Shadow Chancellor added: "If David Cameron is going to go out there and fight this one, we will be supporting him on that."
The court acknowledged that it was up to national authorities to decide exactly who can vote from jail - but denying the right to all inmates indiscriminately is illegal.
In response to Mr Balls' statement, Juliet Lyon, director of the Prison Reform Trust, said: "It's unedifying to see politicians scrambling to stop people in prison acting responsibly and doing their civic duty by voting in elections.
"The punishment of imprisonment is loss of liberty not loss of identity and all other rights and responsibilities."