UK Prime Minister David Cameron entered the final stretch of negotiations to help keep his country in the European Union on Wednesday ahead of a crunch summit, after the European Commission's chief insisted a so-called Brexit is not an option.

Downing Street said there "are still details to be nailed down" after Cameron held talks with EU figures to win them over to a package of changes he says the United Kingdom must get to avoid leaving the bloc.

Wednesday's newspapers in the UK reported that part of the plans, to curtail benefits for migrant workers, had been rebuffed by Eastern European countries the Czech Republic, Hungary, Poland and Slovakia.