The UK's ambassador to the European Union, Sir Ivan Rogers, has resigned just months before tough Brexit negotiations are expected to begin.
The move comes after Sir Ivan, who was set to remain in the post until November, provoked controversy when he privately warned the Government that a post-Brexit trade deal could take a decade to finalise, and that even then may fail to get ratified by member states.
Sir Ivan told ministers in October that other EU members believe a trade deal with the UK may not be hammered out until the early to mid-2020s.
He also said European leaders believe the Brexit deal is likely to be a free trade arrangement rather than continued single market membership.
Sir Ivan's warnings relating to the potential timeframe for a Brexit trade deal are believed to have caused relations to strain with some members of the Government, and his advice prompted some pro-Leave Tory MPs to label him a "gloomy pessimist".
Labour former shadow foreign secretary and chairman of the Commons Brexit Committee Hilary Benn told the BBC the resignation was "not a good thing".
Mr Benn said it was important to maintain continuity during any hand-over period as Sir Ivan steps down from the key role.
"I think that it means that the Government will have to get its skates on to make sure there is a replacement in place so he or she can work with Sir Ivan in the transition, the handover," he said.
"But the hard work is going to start very soon, because if Article 50 is triggered, as the Government says it wishes to, by the end of March, then negotiations will probably begin shortly thereafter.
"And having a handover in the middle of that, depending on when exactly he goes, is not ideal."
Leave.EU chairman Arron Banks said: "This is a man who claimed it could take up to 10 years to agree a Brexit deal. He is far too much of a pessimist and yet another of the establishment's pro-EU old guard. He has at least done the honourable thing in resigning.
"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."
Nigel Farage told the Press Association the resignation should be the first of a world-wide wave by British ambassadors, as he called for a "tough Brexiteer" to take up the EU post.
"I think it would be appropriate if a lot more people in that position, British ambassadors, left. The world has changed.
"The political establishment in this country and the diplomatic service just doesn't accept the vote."
Asked if this was a major crisis for Prime Minister Theresa May, Mr Farage said: "It shouldn't be. She should welcome it with open arms and put a firm Brexiteer in the position.
"Somebody who recognises that the world has changed, somebody tough, so we can get on with Brexit."
Responding to a suggestion from Ukip MEP Gerard Batten that Mr Farage should be appointed British ambassador to the EU, the former party leader said: "That's very sweet of him. It would be lots of fun, but it's never going to happen."
Nick Clegg, the former deputy prime minister and Liberal Democrat European Union spokesman, said: "The resignation of somebody as experienced as Sir Ivan Rogers is a body blow to the Government's Brexit plans.
"I worked for Ivan Rogers in the EU twenty years ago - then he worked for me and the rest of the coalition government several years later.
"Throughout all that time Ivan was always punctiliously objective and rigorous in all he did and all the advice he provided.
"If the reports are true that he has been hounded out by hostile Brexiteers in Government, it counts as a spectacular own goal.
"The Government needs all the help it can get from good civil servants to deliver a workable Brexit."