David Cameron has admitted that the referendum on whether Britain should stay in the European Union is likely to take place before the UK's new membership terms have been implemented in a new EU treaty.

His officials insisted that the new deal for Britain he intends to strike with its EU partners would be “crystal clear” to the public and have “legally binding” guarantees that would amount to an “irreversible lock.”

Mr Cameron has promised an in/out referendum by 2017 and it could be held in October next year. Some changes he is seeking will require the EU’s governing treaty to be rewritten – such as Britain opting out of the EU’s commitment to “ever closer union” and denying tax credits to new EU migrants for four years.

But there is no chance of the EU approving a new blueprint by 2017. So Britain’s new deal would be added to the next treaty, likely to bring in reforms to the Eurozone. But Brussels officials say that work might only start on that in 2017 and could take until 2025 to complete. Another cause of delay is that countries such as Ireland, France and Denmark would need to approve the new treaty in a referendum.

Eurosceptic Tory MPs Show all 7 1 /7 Eurosceptic Tory MPs Eurosceptic Tory MPs Owen Paterson Formerly a cabinet minister, Owen Paterson is now free to make his opinion known on the backbenchers. On the subject of Europe, he does so regularly – claiming recently that the EU referendum was “rigged” in favour of staying in Getty Eurosceptic Tory MPs John Redwood A longstanding eurosceptic, Mr Redwood warned last year that businesses that spoke out in favour of EU membership would be punished at the check-outs by anti-EU Getty Eurosceptic Tory MPs Bill Cash Awkward squad rebel Bill Cash said last year that he thought the EU had become an undemocratic, German-dominated project. “An increasingly assertive German Europe is at odds with British national interests,” he wrote in the Daily Telegraph Getty Eurosceptic Tory MPs Philip Davies From the Conservative party’s hard right wing, Philip Davies has been a longstanding critic of the EU. He founded the Better off Out campaign and is so eurosceptic that Ukip decided not to stand a candidate against him in 2010 because they agreed with him Rex Eurosceptic Tory MPs Nadine Dorries Outspoken Tory MP Nadine Dorries has previously advocated an alliance with Ukip. At the height of the Greek crisis in 2013 she said that the EU was “dying on its feet” Rex Eurosceptic Tory MPs Liam Fox The former defence secretary is a central figure on the right wing of the Conservative party. He’s long put pressure on David Cameron over EU negotiations Getty Eurosceptic Tory MPs Zac Goldsmith A socially liberal eurosceptic, Goldsmith was one of the founding members of the People’s Pledge campaign to get MPs to sign up for an EU referendum. His father ran the Referendum Party, a precursor to Ukip Getty

Conservative Eurosceptics will attack the delayed treaty change as a “post-dated cheque” that could bounce. They are worried that the European Court of Justice, which enforces the EU's rules, could “unpick” Mr Cameron’s agreement with his fellow EU leaders before they are entrenched by a treaty.

John Redwood, the former Cabinet minister, said:"We should never trust what the EU says. They change their mind more often than I change my shirts, which is very often.”

In January, Mr Cameron said he would demand "proper, full-on treaty change" in his negotiations with the EU.

British officials dismissed the possible threat caused by the timelag, insisting that ratifying the treaty after a UK referendum would be a “technicality” and a “formality.”

At a summit of EU leaders in Brussels, Mr Cameron spelt out his proposals over a working dinner of asparagus, sea bass and strawberries. The meeting was expected to ask officials to look at Britain's reforms in depth – including the need for treaty change — and to report back to the leaders’ summit in December.

The Prime Minister was expected to tell his counterparts that for many years there had been “widespread unease” in Britain about its EU membership. It was important to achieve his reforms so that people could be convinced that the 28-nation bloc served their interests, he argued.

His officials said that while Mr Cameron “ruled nothing out,” he was not threatening to recommend an Out vote in the referendum if he did not get his way.

David Cameron is seeking a ‘new deal for Britain’ at the European summit in Brussels (AFP)

There were early signs that the Prime Minister faces an uphill struggle. Donald Tusk, President of the European Council - the 28 national leaders - was happy to discuss Britain's concerns, but warned: "One thing should be clear from the very beginning - the fundamental values of the EU are not for sale and so are non-negotiable."

Martin Schulz, President of the European Parliament, said: "Solutions in the EU usually are not brought about by one member state making demands and expecting the others to deliver."

The Queen has unintentionally been drawn into the referendum debate after she said in a speech in Berlin on 24 June that “division in Europe is dangerous and we must guard against it in the West as well as the East of our continent.”

Buckingham Palace denied that the monarch was straying into politics but Nigel Farage, the Ukip leader, said: “The Queen has been very poorly advised. It’s been a rotten week for the monarchy after the row with the SNP too. She must not let the government pressure her into saying things that appear to influence the referendum.