Treaty changes to reform the UK's relationship with the 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 before they go to the polling booths.
The development 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 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.
He was due to restate his position during a working dinner at the European Council meeting.
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.
And 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.
Officials played down concerns that any promised reforms may be blocked after their agreement by referendums in other EU nations, such as Ireland which has a convention that treaty changes are subject to a public vote.