By Conor Humphries and Padraic Halpin
DUBLIN (Reuters) - It will be difficult but not impossible for officials renegotiating Britain's EU membership to have a framework ready for substantial discussion at next month's European Union summit, the EU's chief negotiator said on Wednesday.
Prime Minister David Cameron is trying to win concessions from other leaders ahead of a referendum on British membership promised by the end of 2017 amid opinion polls that show British perceptions of the EU are changeable.
There had been some hope that at least the bulk of a deal might be ready for the European Council to endorse at its next meeting on Dec. 17-18 but officials and diplomats fear there may not even be a narrowing of differences by then.
"No doubt it's not impossible but given the complexity of some of these issues and the relatively recent date on which the Prime Minister sent his (renegotiation) letter, it's not going to be easy," Jonathan Faull, himself a Briton, told an Irish parliamentary committee meeting.
"We'll see where we are before the European Council meeting on December 17. If that's not possible we will go into next year and the next scheduled European Council is in February and that gives us a few more weeks to work on these very tricky issues."
Faull, a veteran EU administrator, was appointed by EU chief executive Jean-Claude Juncker in June to head a task force dealing with reforms demanded by Cameron.
Asked if a compromise could be found on Britain's wish to be excluded from the principle of "ever closer union", Faull said: "I don't think that's impossible."
But Faull said a more contentious proposition that people coming to Britain from the EU must live in the country for four years before qualifying for state benefits was being complicated by continuing reforms to the British welfare system and would be the most difficult aspect of the talks.
"That looks very like discrimination [to some people], so poses very serious problems under single market rules," Faull added in a speech to an EU think tank.
"We have a significant political legal task ahead of us about how you can run your social security system in a single market. I can't pretend that that is easy. I think in fact it is the most difficult of all of these issues."
Faull said the European Commission did not have a 'Plan B' if Britain voted to leave the EU but warned of grave consequences in the event of a "Brexit".
"I hope it doesn't happen, we are working hard to try to make sure it doesn't happen, but there is no doubt that it would have very considerable consequences for our continent and for our union," he said.
"Some of those will perhaps be predictable and some of them will be unpredictable as Europe reacts to this new and unprecedented development."
(Editing by Catherine Evans)