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European Commission president Jean-Claude Juncker said he had blocked eurocrats from holding “negotiations behind closed doors in smoke-filled rooms”.

It comes amid a growing row over when Britain will trigger Article 50 of the Lisbon Treaty, the formal process by which the UK will leave the bloc.

David Cameron has said he will leave the decision over when to invoke Article 50 to the next prime minister, due to be elected by September 2.

But Mr Juncker warned there would be a “presidential ban” on European commissioners talking to UK Government officials until Article 50 is invoked.

He added: “I would not like the idea that there could be some sort of negotiations behind closed doors in smoke-filled rooms.

“There can be no preliminary discussions.”

Speaking to the European Parliament in Brussels, he told MEPs: "No notification, no negotiation".