The proposals to introduce directly elected police commissioners in each force area are due to be debated by MPs today.
The plans have been criticised for costing an estimated 100 million 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: "People will rightly be angry that the government can find 100 million 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."