BRUSSELS (Reuters) - EU chief executive Jean-Claude Juncker pledged on Wednesday to work for fair new membership terms for Britain but warned David Cameron to respect Europe's interests as well, saying "it takes two to tango".

The European Commission president, who will meet the British prime minister over lunch on Thursday ahead of a summit of EU national leaders, repeated in a speech to the European Parliament that he wanted "a fair deal with Britain".

After weeks of technical talks between British and EU officials to set a framework for political negotiations, Juncker added: "I can't say that huge progress has been achieved. I can't say that nothing has been achieved. But to tango it takes two.

"I am not a splendid dancer, but at least I know the rules which have to be observed by others. And it needs two to tango. And so we have to dance and our British friends have to dance."

EU officials and diplomats have voiced impatience that, since Cameron confirmed a referendum on EU membership after his re-election in May, London has not set out a precise set of proposals for legislation. British officials say the process is on track for political talks by December.

At Thursday's EU summit, the chairman will brief leaders on how negotiations on EU reforms Cameron wants will be handled before another summit discusses the issue in mid-December. British officials say there is no deadline for concluding talks beyond the fact the referendum will be held by the end of 2017.

Draft conclusions of Thursday's meeting, seen by Reuters, read: "The European Council was informed about the process ahead concerning the UK plans for an (in/out) referendum. The European Council will revert to the matter in December.

EU leaders have made clear they do not want lose the bloc's second-biggest economy but have warned that some of Cameron's demands, notably on curbing immigration by EU citizens, will be very hard to meet without breaking basic Union principles.

"For making sure," Juncker told the parliament in English, "I am 150 percent in favour of having Britain as a constructive member state of the European Union. We need Britain. Personally I do think that Britain needs the European Union."

(Editing by Hugh Lawson)