Treaty changes to reform the UK's relationship with the European Union (EU) may not be in place by the time of the in/out referendum David Cameron has promised by the end of 2017, British officials have confirmed.
But they insisted the Prime Minister will secure "legally-binding and irreversible" assurances that EU law will be changed to incorporate the reforms, which will be "crystal clear" to voters.
Labour accused Mr Cameron of being prepared to ask voters to go to the polls on the basis of a "post-dated cheque" and accused the PM of retreating in the face of opposition from other EU leaders to his demands.
Meanwhile, Nigel Farage suggested that the Prime Minister had dragged the Queen into the EU row after she warned of the dangers of "division in Europe" in a speech during her state visit to Germany.
"She must not let the Government pressure her into saying things that appear to influence the referendum," said the Ukip leader.
" She is, after all, the head of the Commonwealth as well as head of the UK, and there is a potential conflict of interests there."
The developments came as a Brussels summit gave the green light to the opening of formal talks on the renegotiation in which Mr Cameron hopes to secure a package of reforms which will allow him to recommend a Yes vote to stay in the EU.
Mr Cameron said it was a "significant milestone" to have the renegotiation on the agenda at the gathering of the EU's 28 national leaders, even though the issue was being discussed only briefly in a two-day meeting dominated by Greek debt and the Mediterranean migrant crisis.
In individual talks with all of the other 27 EU leaders ahead of the summit, the Prime Minister made clear that he believes treaty change will be needed to deal with issues including EU migration into the UK, a British opt-out from the goal of "ever-closer union" in Europe, greater powers for national parliaments and protections for countries - like the UK - which are not members of the single currency.
Addressing them over dinner in Brussels, he warned of the "widespread unease" in Britain over the terms of the UK's membership and said he was determined to bring the issue to a public vote following renegotiation.
It was important that British voters felt the EU was ready to serve their interests and respond to their concerns, he added.
In bilateral talks after arriving in Brussels, he secured agreement from European Council president Donald Tusk that talks should begin between British and EU officials on the substance of changes Britain is demanding.
A leaked draft of the communique expected to be agreed by the European Council tomorrow indicated that these "technical" talks on Mr Cameron's proposals will take at least six months, before the leaders discuss them once more at a summit in December.
British officials acknowledged that a final decision on the shape of the reform package may not be agreed even then, insisting that the talks would be driven by the substance of the discussions rather than an "arbitrary" timetable.
They insisted that there was no change in Mr Cameron's position on the timing of treaty changes, insisting the UK had always assumed they would not be in place by the time of the referendum, because of the lengthy process of ratifying them in national parliaments across the EU.
"There will be a process that will be needed to bring changes into force but we will get agreement on the treaty change before the referendum. Will it be crystal clear and binding at the point it goes to the British people? Yes," said one official.
UK officials played down concerns that any promised reforms may be blocked after their agreement by referendums in other EU nations, such as Ireland, which observe a convention that treaty changes are subject to a public vote.
Shadow foreign secretary Hilary Benn said: "All year the Prime Minister has been saying that change to the treaty was a definite requirement and yet now, faced with entirely predictable opposition from other member states, he is signalling retreat while pretending that all he ever wanted was a post-dated cheque.
"The Prime Minister has made a mess of this. He should have known what the position of other countries would be, but because he did not prepare the ground for the second time in a few months he has been forced to admit that he can't get what he wants.
"The same pattern repeats itself over and over again with the Prime Minister and Europe. He marches his troops up to the top of the hill and then he has to march them down again.
"He is so intent on keeping his eurosceptic backbenchers at bay that a calm negotiation of what is in the country's interests - remaining a member of the European Union - comes second."
As leaders gathered in Brussels, there were signs of the resistance Mr Cameron will face in his drive to reshape the UK's relationship with the EU.
Mr Tusk 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."
European Parliament President Martin Schulz added: "Solutions in the EU usually are not brought about by one member state making demands and expecting the others to deliver."
In his discussions with fellow EU leaders, Mr Cameron will have come to the realisation that "treaty change doesn't go without saying, that there is quite some resistance to changing the treaties", said Mr Schulz, who warned the process could take up to four years.
He suggested that the Prime Minister's chances of success might depend on whether he was seeking to win over the British people or his own Conservative colleagues.
"Who is the audience of the Prime Minister?" asked Mr Schulz.
"If it is the Tory group in the Parliament, I think we are facing a lot of problems. If it is the British people to convince, then you can present successes you can achieve here also as a success to win the majority of the people."
European Commission President Jean Claude Juncker appeared unsure what reforms Mr Cameron was seeking, asking reporters: "What does he want?"
The changes to welfare entitlements of EU migrants sought by the PM were "a matter for national lawmakers", he said.
Belgium's finance minister Johan Van Overtveldt said a British opt-out from ever-closer union would be " not easy".