A group of prisoners is threatening to sue the Government for compensation if their members are prevented from voting in the general election.
The Association of Prisoners want at least £1,000 for every offender in England and Wales refused voting rights.
Five years ago the European Court of Human Rights ruled it was illegal for ministers to deny voting rights to all prisoners.
Since then the Government has held two public consultations on the issue but has not changed the law.
Prison reform groups made a formal complaint to the Council of Europe accusing ministers of using delaying tactics.
Last year Justice Minister Michael Wills confirmed it was "unavoidable" that some inmates would be given voting rights.
A policy paper published in April suggested prisoners serving sentences of up to four years could be allowed to vote.
That would mean giving voting rights to around a third of the 84,000 currently in custody.
Ben Gunn, General Secretary of the Association of Prisoners, said the Government had defied the court for too long.
"I deplore the Government's ineptitude. This Government that has locked up more people than any in UK history has the temerity to claim moral authority, whilst acting in bad faith in defiance of implementing the court's judgment," he said.
"Criminals will doubtless be glad to know that the rule of law is an optional extra. We will certainly be glad for the compo."
Frances Crook, director of the Howard league for Penal Reform, accused ministers of using the issue as a "political football".
"Prisoners are absolutely right to fight for their right to vote and I hope the change comes in before the next election," she said.
"Ministers have been avoiding this issue for far too long in a bid to look tough on crime, but prisoner voting shouldn't be used as a political football.
"Losing one's liberty is punishment in itself. The Government has a duty to encourage civic responsibility, particularly amongst marginalised groups."