David Cameron might have to make do with a "irreversible lock" and "legally binding" guarantees rather than full-on treaty changes before Britain goes to an In/Out EU referendum.
According to the BBC, Cameron has accepted that there might not be EU treaty changes before the referendum, which is due to take place before the end of 2017. However, Downing Street stressed that the Prime Minister remained committed to "proper, full-on treaty change".
A senior British official told The Guardian that the prime minister is confident that he will embed his reforms in a "legally binding and irreversible" process that will involve a revision of the Lisbon treaty. But this could take years to complete, given that another 27 states are involved.
The official said: "There will be political agreement at 27 that these changes will be done. Then there is a process of ratification of those which can take a long time because it needs to be ratified in all 27 countries.
"So there will be a process that will need to bring the changes to the treaties into force. Are we absolutely clear that the reforms we are seeking will require treaty change and will need agreement on that treaty change before that referendum? Yes we absolutely are."
Another official added: "Will it be crystal clear and binding at the point that goes to the British people? Yes it will."
Eurosceptics dismiss 'guarantees'
The report has already sparked anger among the Conservative eurosceptic awkward squad.
Tory MP John Redwood said: "We should never trust what the EU says. They change their mind more often than I change my shirts, which is very often."
Nigel Farage, the leader of the United Kingdom Independence Party, was scathing, calling the move a "post-dated cheque".
He added: "Post-dated cheques can bounce and one suspects that any post-dated cheque that was given to the Brits would be given by presidents and prime ministers in office now.
"By the time it came to be honoured there would be different prime ministers and different presidents who have been elected on a ticket saying we won't honour this note anyway. It doesn't work on any way you look at it."
He told the BBC that the prime minister was "accepting the inevitable" that there would be no treaty change, adding there "will be nothing really fundamental in Britain's renegotiation of its membership [of the European Union]".
"This doesn't look like a man on a mission who is determined to achieve something whoever he upsets. This looks like somebody who is now setting his ambitions for the change of our relationship incredibly low."
But former Conservative Home Office minister Damian Green, a pro-European MP, said it was "perfectly sensible for the prime minister to leave his options open" to get the results he wanted.
"It's obvious that a treaty change would require referendums in other countries so it would be a very long, drawn out process. You can see why other countries won't welcome that."