LONDON (Reuters) - British Prime Minister David Cameron will give his strongest warning this week that he might back Britain leaving the EU unless other European leaders agree to his demands for reform of the bloc, media reported on Saturday.
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 the media reports.
"As I have said before – I rule nothing out."
Cameron has faced criticism both at home and abroad 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.
The reports said Cameron's letter to Tusk would 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.
While Cameron has never ruled out campaigning to leave the EU if he failed to secure any agreement, media said the tone of the speech was his strongest assertion to date that the status quo was unacceptable.
However, in his speech Cameron will also repeat that he wanted Britain to remain in the 28-nation bloc, which it joined in 1973, and was confident a deal could be struck which would suit Britain and its partners.
He will also deliver a strong message to both those campaigning for a British exit and for those who want to stay in the EU regardless of whether reform could be achieved.
"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, according to the reports.
"Those who think Britain should just leave now also need to think hard about the implications of their arguments. 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 the bloc for remaining versus leaving has narrowed in recent months.
(Reporting by Michael Holden; Editing by Alan Crosby)