The government has resisted calls to allow prisoners the vote
The general election will break the law unless the ban on prisoners voting is lifted, a pressure group has warned.
Barred From Voting, which includes prison governors, penal reformers, politicians and church leaders, says new legislation is needed immediately.
In December, the Council of Europe's Committee of Ministers said the ban meant the election risked breaching the European human rights convention.
The Ministry of Justice said the issue remained under "careful consideration".
In 2005, the Grand Chamber of the European Court of Human Rights declared that it was unlawful to deny all sentenced prisoners voting rights in UK elections.
Since then, the government has consulted twice on voting reforms, but there is no sign that any changes will be in place by the general election.
The Committee of Ministers at the Council of Europe expressed "serious concern" that there was a "significant risk" the election might fail to comply with the European Convention on Human Rights.
The Barred from Voting group has amplified those concerns, but says there is still time for ministers to introduce measures to enfranchise sentenced prisoners.