By Michael Holden and William James
LONDON (Reuters) - British Prime Minister David Cameron will this week give his strongest warning yet that he might back Britain leaving the EU unless other European leaders agree to his demands for reform of the bloc.
Cameron is due to outline British demands for renegotiation of its European Union membership terms in a letter to the President of the European Council, Donald Tusk, to be published on Tuesday.
In a speech the same day, he will say that if no deal can be reached, he could back a British exit when an in/out referendum is held before the end of 2017.
"If we can't reach such an agreement, and if Britain's concerns were to be met with a deaf ear, which I do not believe will happen, then we will have to think again about whether this European Union is right for us," Cameron will say, according to advance extracts of his speech.
"As I have said before -- I rule nothing out."
Cameron has faced criticism both at home and abroad -- including from his own euroskeptic backbenchers -- for not spelling out details of the concessions he is seeking from other European leaders with detailed discussions expected to accelerate before a summit next month.
Cameron's letter to Tusk is expected to include demands such as barring in-work benefits for EU migrants for four years, an exemption from any closer EU integration, and more powers for national governments to block EU legislation.
British Foreign Secretary Philip Hammond said the letter would draw together changes the government has previously said it is seeking but would not demand specific legislative measures.
"We don't want to be excessively prescriptive at the beginning of a discussion," Hammond told the BBC, saying that there were a range of ways to achieve most of Britain's objectives.
"This letter is not the end of the process, it's the beginning of the process," he said.
The Vote Leave campaign, which wants Britain to exit the bloc, said Cameron's reform agenda lacked ambition, calling it a "dishonest gimmick".
"We expect Cameron to get what he's asking for, but what he's asking for is trivial," the campaign's director, Dominic Cummings, said.
Cameron's office said that Britain will begin a fresh round of meetings as the renegotiation enters a new "intensive" phase, with senior representatives of member states invited to Brussels to discuss the letter along with European Council officials.
While Cameron has never ruled out campaigning to leave the EU if he failed to secure any agreement, the tone of the speech will be his strongest assertion to date that the status quo is unacceptable.
However, Cameron will also repeat that he wants Britain to remain in the 28-nation bloc, which it joined in 1973, and is confident a deal can be struck to satisfy Britain and its partners.
He will also deliver a strong message to those on both sides of the debate over a British exit.
"Those who believe we should stay in the EU at all costs need to explain why Britain should accept the status quo. I am clear that there are real problems with this," he will say.
"Those who think Britain should just leave now also need to think hard about the implications ... What would being outside the European Union mean for our economic security?"
Opinion polls show most Britons favor staying within the EU although support for remaining versus leaving has narrowed in recent months.
(Editing by Alan Crosby and David Goodman)