Opponents of Britain entering the single currency have criticised the idea that more than 700,000 EU citizens living in the UK might be allowed to vote in a referendum.

Ian Davidson, leader of Labour's No campaign, warned: "This is the start of attempts to fix the vote.

"I am greatly concerned that the government will try to bias any euro referendum by manipulating not only the date, but also the rules, the question, the spending limits and now even the electorate."