This correction was published in the Observer's For the records column, Sunday November 16 2008

The headline above is incorrect. It is parliament's Joint Committee on Human Rights that says the law must be changed to comply with a European Court of Human Rights ruling that prison inmates should be given the right to vote. The European Court of Human Rights is not a European Union institution.

The government must give prisoners the right to vote or the next general election will be illegal under European law, ministers have been warned by parliament's influential Joint Committee on Human Rights.

The committee's conclusion threatens a constitutional crisis for Labour, which has tried to bury the issue ever since the European Court of Human Rights ruled in 2005 that inmates should have the vote. The committee, comprising six MPs and six peers, has written to the Ministry of Justice saying the government must urgently change the law so that the majority of Britain's 84,000 prisoners are given the right before the country next goes to the polls.

'A legislative solution can and should be introduced during the next parliamentary session,' it states. 'If the government fails to meet this timetable, there is a significant risk that the next general election will take place in a way that fails to comply with the convention and at least part of the prison population will be unlawfully disenfranchised.'

In addition to convicted prisoners, those banned from voting include members of the Lords, anyone found guilty of election corruption within the past five years and people with learning disabilities or a mental illness deemed incapable of making a reasoned judgment.

Being hauled before the European Court of Human Rights for failing to comply with the convention is an unpalatable prospect for the government. But introducing a law allowing jailed offenders to vote is likely to spark a furious row that the government's opponents could exploit to their political advantage.

'The idea that our entire democratic process could be overturned in this way is ludicrous,' said shadow justice secretary Nick Herbert. 'Claiming sweeping new entitlements for prisoners is a classic example of over-reaching human rights laws which have lost sight of the importance of balancing responsibilities. Parliament should make these decisions, not European judges.'

The government originally said it would consider the issue of prisoners' voting rights in a two-stage consultation that was supposed to have been completed in January 2008. Ministers said a new law would follow after May 2008.

But a joint committee member attacked the Justice Ministry for dithering on the issue. 'The government cannot pick and choose which human rights treaty obligations it fulfils for party political reasons or just because it feels an issue is not populist enough,' said Evan Harris, a Liberal Democrat MP. 'Gordon Brown is going soft on human rights. There is every chance that this country may be in breach of international law if the government doesn't have the courage to act before the next general election.'

The Prison Reform Trust, which campaigns on behalf of prisoners, has written to the Justice Secretary, Jack Straw, asking why the government was delaying the legislation. 'This mean-minded, foot-dragging approach... calls into question the government's commitment to social inclusion, citizenship and human rights,' said Juliet Lyon, the trust's director.

The European ruling stipulates only that a blanket ban on prisoner voting is unlawful, suggesting that certain segments of the prison population - such as murderers Ian Huntley and Rosemary West - will still not be entitled to vote. Legal experts believe the government may seek to limit the prisoners who can vote to those who have been rehabilitated and are considered 'good citizens'. But the judgment, made in response to a case brought before the European Court of Human Rights by a British prisoner called John Hirst, suggests imposing this qualification may be difficult.

Following the Hirst ruling, several states passed laws giving prisoners the right to vote. In 2006 Ireland passed legislation allowing all prisoners to vote by post in the constituency where they would live if they were not in prison.

A spokeswoman for the Ministry of Justice said: 'Prisoner voting rights is a sensitive and complex issue, and we need to look very carefully at what the right approach to prisoner enfranchisement in the UK is and at how it would be implemented.'

· This article's headline was amended on Wednesday November 12 2008 from 'Ban on votes for prisoners is illegal, says EU' to 'Ban on votes for prisoners is illegal, says parliamentary committee'.