SEMI-DETACHED

Britain is already the EU's most semi-detached member, having opted out of joining the euro single currency, the Schengen zone of passport-free travel and many areas of police and judicial cooperation.

Many leaders said they felt they were at a historic turning point for European integration.

No country has ever voted to leave the Union. Britain is the EU's second-largest economy and one of its two permanent members on the UN Security Council. Its exit would end the vision of the EU as the natural home for European democracies and reverse the continent's post-World War Two march toward "ever closer union".

Belgium, the most federalist of EU members, was pressing for a clause to ensure the deal with Britain would automatically cease to exist in case of a vote to leave - to make sure there was no possibility of a second renegotiation.

The issue has divided Cameron's Conservative Party for decades, crippling his 1990s predecessor John Major and bringing down his hero Margaret Thatcher.

Some Conservatives have criticised the reforms he is negotiating in Brussels as trivial, although most senior party figures are likely to join him in campaigning to stay in if he wins the concessions he is seeking.

Before even the final deal with Brussels was done, the BBC said his friend and justice minister Michael Gove would declare his intention to campaign to leave the European Union.

Britain's largely eurosceptic press depicted Cameron as begging or pleading, the Daily Mail describing him as "rattled".

"Shambles as embattled PM's deal is watered down," a front-page headline read over a picture of an anxious-looking Cameron.