European Commission President Jean Claude Juncker urged Britain to drop a veto on EU spending imposed last week, saying this would help ease negotiations over the country's withdrawal from the European Union.

"The UK is currently blocking a decision, and it would be desirable and it would facilitate the beginning of the negotiations if the UK were to be able to withdraw the reserve it has entered," Juncker told a news conference after a summit of EU leaders to agree Brexit negotiating demands.

British officials say London withheld approval on Wednesday for some €6 billion euros of EU spending, notably to tackle migration problems in the Mediterranean, because of a policy of holding off on politically sensitive decisions during "purdah", a period of weeks before a June 8 British election.

Juncker also said some politicians in Britain do not understand the difficult talks ahead, notably when they say that the EU should offer an immediate guarantee of expats' rights.

"I have the impression sometimes that our British friends, not all of them, do underestimate the technical difficulties we have to face. This single and not simple question of citizens' rights is, in fact, a cocktail of 25 different questions which have to be resolved," he said.