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 Rogers, 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.
He 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.
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Rogers'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 handover period as Rogers 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 didn't mince his words and was glad to see Rogers step down.
"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," Banks said.
