Prisoners could be allowed to vote for local police chiefs from their cells in future, it was claimed yesterday.
Coalition plans to introduce directly-elected police commissioners in each force area are due to be debated by MPs today.
The proposals have been criticised for costing an estimated 100m at a time when budget cuts are being imposed on forces across the country.
But Shadow Home Secretary Ed Balls has now raised concerns that the legislation does not rule out letting convicts take part in the ballots.
The European Court of Human Rights (ECHR) has given Britain nine months to remove its total ban on prisoners voting, and ministers have reluctantly accepted that they must comply with the ruling.
Mr Balls said: "At a time when the Government is slashing police funding by 20 per cent people will rightly be angry that the Government can find 100m for a new set of politicians.
"But to make matters worse we're set to be the first country in the world, alongside two small states in America, where convicted criminals can vote for police chiefs from inside their prison cells.
"Ministers have failed to include anything in the legislation to rule this out. It is madness and we will oppose these plans. They risk politicising the police at huge cost to the public just when thousands of police officers are being cut."
Prime Minister David Cameron has said the prospect of giving those serving time electoral rights makes him "physically ill".
However, failure to comply with a 2004 ECHR ruling could cost the Government hundreds of millions of pounds in compensation payouts.
There is speculation the restriction could be kept for people serving life sentences, and judges may be given responsibility for deciding which criminals should be allowed to vote.
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