From: news.bbc.co.uk

Writing in the Daily Mail, Mr Davis said the vote was needed to give a negotiating mandate for a future Tory government to take to the EU.

The referendum question should incorporate aims such as "recovering control over our criminal justice, asylum and immigration policies" and a "robust opt-out of the European Charter of Fundamental Rights", he wrote.

"His challenge will be to find a line that satisfies his party and that he believes a Conservative government could realistically deliver," said our correspondent.

Eurosceptics in his own party have accused Mr Cameron of reneging on a "cast iron" guarantee made in 2007 to hold a referendum on any treaty that emerged from EU talks, if he became prime minister.