Treaty changes to reform the UK's relationship with the EU may not be in place by the time of the in/out referendum, 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 reforms, which will be "crystal clear" to voters before they go to the polling booths by the end of 2017 .
The development came as a Brussels summit gave the green light to formal talks where David Cameron hopes to secure reforms which will allow him to recommend a Yes vote to stay in the EU.
EU leaders say the 28-member bloc is stronger with Britain as David Cameron hopes to push forward renegotiation talks at a Brussels summit.Read the full story ›
President of the European Council, Donald Tusk met David Cameron ahead of the summit today and said the meeting would mark the start of a consideration of British concerns.
But he warned that the EU's fundamental principles were "not for sale and so are non-negotiable".
There are some British concerns we should consider but only in a way which will be safe for all Europe. Today we start this process.
However, one thing should be clear from the very beginning - the fundamental values of the EU are not for sale and so are non-negotiable.
Prime Minister David Cameron said that the European Council summit in Brussels today is a "milestone" for Britain in the process of its renegotiation with the EU.
Today marks a significant milestone in the process of saying it is right for Britain to have this renegotiation and this referendum to address the concerns that the British people have about Europe and to make sure the British people have a final say about whether we stay in a reformed European Union or leave.
But today we're also discussing this vitally important issue of European migration where we need a comprehensive and co-ordinated approach and Britain will play its role.
He said that along with existing aid and rescue efforts in the Mediterranean, his government is also looking at how to tackle criminal gangs that are trafficking people across Africa and into Europe.
"I look forward to discussing what more Britain can do to help with this crisis," he added.
David Cameron plans to complete his one-to-one discussions will all 27 other leaders of the European Union before this afternoon's summit in Brussels.
He will meet with the leaders of Portugal, Bulgaria, Lithuania and Austria.
And he will talk by phone with the leaders of Croatia and Greece - despite the debt crisis in which the Greek Prime Minister is currently wrapped up.
It means by the time David Cameron addresses the EU Council tonight during the leaders' working dinner - they will all, said Downing Street, "know the issues the Prime Minister wants to address".
However, with the EU facing both the Greek and the migrant crisis, Number 10 has been forced to concede that they "recognise" that the "UK renegotiation sits alongside that".
It suggests that many of the 27 leaders outside the UK do not consider the UK's referendum and renegotiation process a priority.
Alan Johnson, leader of the Labour Group's Yes campaign, says leaving the EU could put jobs as well as our rights at risk, while eurosceptic Tory MP John Redwood says far from losing friends, Britain will gain in stature as an independent country.
But who do you agree with? Should we stay or should we go?
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Conservative MP and eurosceptic John Redwood says far from losing friends Britain will gain in stature as an independent country.Read the full story ›
Alan Johnson, leader of the Labour Group's Yes campaign, says leaving the EU could also put jobs as well as our rights at risk.Read the full story ›
Here are 10 things which could change if Britain were to break its 42-year union with Europe.Read the full story ›
The Prime Minister will address leaders of the other EU nations today in a bid to kick-start formal talks on reform of Britain's membership.Read the full story ›