UK's EU envoy quits over 'muddled thinking' on Brexit

He was criticised by Brexit supporters last month for warning that a new deal with the EU could take up to 10 years.

Sir Ivan, whose deputy resigned late last year, argues in his letter that it makes sense to hand over the baton to a new team before exit talks officially begin.

His departure is seen as a blow to prime minister Theresa May's government.

In his resignation letter , Rogers warns that the government lacks the necessary knowledge and technical expertise to conduct the negotiations, and the involvement of the UK permanent representation in the talks is essential.

"Serious multilateral negotiating experience is in short supply in Whitehall, and that is not the case in the Commission or in the Council," he wrote.

"The government will only achieve the best for the country if it harnesses the best experience we have - a large proportion of which is concentrated in UKREP [the UK EU representation] - and negotiates resolutely."

Rogers also made it clear that the UK has no negotiating position six months after the UK's vote to leave the union, and only three months before prime minister Theresa May intends to officially launch exit talks.

"We do not yet know what the government will set as negotiating objectives for the UK's relationship with the EU after exit," Sir Ivan writes.

He encourages his staff to support each other and deliver bad and uncomfortable news to decision-makers.

"Senior ministers, who will decide on our positions, issue by issue, also need from you detailed, unvarnished - even where this is uncomfortable - and nuanced understanding of the views, interests and incentives of the other 27," he says.

"I hope you will continue to challenge ill-founded arguments and muddled thinking and that you will never be afraid to speak the truth to those in power."

His letter sheds uneasy light on how ill-prepared the British government is, just a few months before negotiations are due to begin on the UK's withdrawal from the EU.

Praise

Brexiteers such as Nigel Farage welcomed his resignation, saying that the Foreign Office needed a "complete clear-out".

Richard Tice, a leader of the pro-Brexit Leave Means Leave pressure group, labelled Sir Ivan a "remainer" who "consistently failed to acknowledge the benefits Brexit will bring to Britain".

But others see the senior diplomat's exit as a blow for the forthcoming negotiations.

Former EU trade commissioner Lord Peter Mandelson described Sir Ivan's resignation as “a serious loss”, adding in a statement that "everyone knows that civil servants are being increasingly inhibited in offering objective opinion and advice to ministers".

"Our negotiation as a whole will go nowhere if ministers are going to delude themselves about the immense difficulty and challenges Britain faces in implementing the referendum decision," Mandelson said.

Claude Moraes, a Labour member of the European Parliament tweeted that the resignation "confirms what the 27 feared and the result will be a bad negotiation for the British people".

Charles Grant, director of the Centre for European Reform think tank, called Sir Ivan one of the very few top British civil servants who understood the EU and said his resignation "makes a good deal on Brexit less likely".

The UK government is expected to appoint a new ambassador and deputy ambassador shortly.