Britain must give prisoners voting rights but the UK can limit that right to certain types of prisoners, the European Court of Human Rights rules in a decision causing fury among MPs.
Britain has refused to let any prisoner vote for 142 years and the ECHR decision means the UK must lift its blanket ban and have a bill in place by November to allow prisoners to participate in elections.
"The public will be demanding that the Prime Minister now stands up for British interests and refuses to give convicted prisoners the right to vote," Conservate backbencher Priti Patel said.
Britain doesn't have to let every prisoner vote, however. The Strasbourg judges accepted that UK politicians - who have refused to follow the ECHR orders on voting rights for seven years - have wide discretion to decide which type of prisoner can vote or whether a judge should decide in individual cases.
"The judgement does not require that all prisoners be given the vote and the ECHR has never made that suggestion," said Ian Loveland, a professor of law at City University London. "It is quite likely that a very modest change to the law would ensure that the UK is not in breach of its international law obligations."
Cameron: One prisoner is one too many
The problem with UK law is that it disenfranchises all prisoners, so a mass-murderer serving life and a first-time shoplifter are treated the same, said Mr Loveland.
Still, Prime Minister David Cameron has already told MPs that the thought of allowing even one prisoner to vote is distasteful: "It makes me physically ill to even contemplate having to give the vote to anyone who is in prison."
The ECHR ruling on Tuesday - in a case called Scoppola v Italy - upholds a ruling in which judges decided in favour of convicted British killer John Hirst, who sued Britain and won in 2005 after claiming his human rights were breached. Hirst celebrated by posting a YouTube video of himself singing "I shot the sheriff", drinking champagne and smoking what he described as a "joint" supplied by the local drug dealer.
Blanket ban unlawful
Tuesday's decision is certain to reignite Britain's debate over its right to control its own sovereignty. The fury reached new heights in February when ECHR judges decided UK terrorist suspect Abu Qatada should be released on bail and held under house arrest - at a cost of millions to UK taxpayers - rather than deported to Jordan because his human rights might be breached. (The extremist cleric's second bail application will be heard 28 May.)
The UK's blanket ban on voting by convicted prisoners was supported in a free vote in the House of Commons by 234 votes to 22 and, in his ECHR submission, Mr Grieve called on judges to return powers to politicians over "the culture of their own particular state".
"Many members of the public as well as MPs are opposed both to prisoners being allowed to vote and to 'interference' by the Strasbourg's court in domestic matters," said Susan Easton, a barrister and senior tutor at Brunel University Law School.
"However, defenders of prisoners' voting rights would argue that there are no security risks involved," Ms Easton said. "Voting rights are fundamental rights rather than privileges to be earned, and treating prisoners as citizens may be an important element in their rehabilitation."