LONDON – Britain’s ambassador to the European Union has resigned, adding uncertainty to the Brexit process less than three months before the UK is due to trigger its departure negotiations.

Ivan Rogers, a highly-regarded diplomat who had been due to end his four-year stint in October, on Tuesday stepped down as London prepares to invoke Article 50, which starts a two-year countdown to Britain leaving the EU.

Rogers came under fire last month for saying it could take 10 years for Britain to conclude a trade deal with the EU.

The government insisted, though, that he was only reporting back what was being said in European capitals.

"Sir Ivan Rogers has resigned a few months early as UK permanent representative to the European Union," a British government spokeswoman said.

"Sir Ivan has taken this decision now to enable a successor to be appointed before the UK invokes Article 50 by the end of March. We are grateful for his work and commitment over the last three years."

London is set to appoint a new ambassador and deputy ambassador shortly.

Rogers headed UKRep, the office which represents Britain in negotiations that take place in the EU.

In a resignation email to UKRep staff, he urged colleagues to provide British ministers with their "unvarnished" understanding through Brexit negotiations -- "even where this is uncomfortable".

"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," Rogers said.

He also criticised the British government for its short supply of "serious multilateral negotiating experience" in London and said the structure of the UK’s negotiating team "needs rapid resolution".

Concerns over timing

"His resignation is not a surprise for those who work with him," a European diplomat said.

"He was very competent, but not convinced by the Brexit decision and the British government line, leading the UK into an area of dangerous uncertainty."

In the June 2016 referendum, 52 per cent voted for Britain leave the EU.

Triggering Article 50 will start a two-year countdown after which Britain will leave all the institutions and the single market unless alternative arrangements have been agreed.

Hilary Benn, who chairs parliament’s Brexit scrutiny committee, told BBC radio the resignation was "not a good thing".

"The hard work is going to start very soon," he said.

"And having a handover in the middle of that, depending on when exactly he goes, is not ideal."

Rogers had been in his post since November 2013, having previously served as prime minister David Cameron’s Europe adviser since 2011.

Aled Williams, the former spokesman for Britain’s EU embassy, said Rogers’ departure was a "big loss" to the Brexit negotiations.

"Sir Ivan never sugar-coated his advice: had the credibility to tell his political bosses how he saw it in Brussels," he said.

The mild-mannered Rogers is widely respected in Brussels where he is known as a vastly experienced operator.

Critics accuse of him of being a europhile, but European diplomatic sources described him as being a realist.

British diplomatic sources at the time of the leaked "10 years" comments voiced suspicions that he may have been knifed in the back by pro-Brexit forces who wanted him out.

UK ministers ’delude themselves’

Peter Mandelson, a British former trade commissioner for the EU, said Rogers was "second to none" in terms of knowledge and experience of Brussels.

Diplomats were being "increasingly inhibited in offering objective advice", he added.

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

But Arron Banks, who chaired the Leave.EU campaign group in the referendum, said Rogers was a "pessimist" from the "pro-EU old guard".

"It’s time now for someone who is optimistic about the future that lies ahead for Brexit Britain. Enough talk, we need to get on with getting out," he said.

Brexit figurehead Nigel Farage also welcomed the resignation, saying the Foreign Office needed a "complete clear-out".

The office of Michel Barnier, the European Commission’s chief Brexit negotiator, declined to comment. – AFP