Prime Minister David Cameron is said to be “delighted” with the way EU renegotiation talks are going, following a two-day summit for all member states in Brussels.
Having made a 10-minute pitch to fellow EU leaders, he said the process of “reform and renegotiation” of the UK's membership was “properly under way”.
"People always say to me these things aren't possible.... Once again we have proved we will get them done.”
Cameron has promised to hold an in-out referendum on membership by the end of 2017, but before then he wants to re-establish the terms of the UK's relationship with the rest of the bloc.
Changes he is seeking include an opt-out on the EU ambition to achieve an “ever closer union”, and greater parliamentary sovereignty to allow the UK to block proposed EU legislation.
European Council President Donald Tusk said while he will listen to the UK's wishes, certain points were “non-negotiable":
One thing should be clear from the very beginning. The fundamental values of the EU are not for sale and so are non-negotiable. We should consider British concerns, but only in a way which will be safe for all Europe.
EU leaders will be presented with the results of the talks at a summit in December.