The UK’s ambassador to the European Union-EU, Sir Ivan Rogers, has resigned.
Sir Ivan, who was appointed to the role of permanent representative by David Cameron in 2013, had been expected to play a key role in Brexit talks expected to start within months.
The Foreign Office has not given reasons for his departure.
Last month, it was revealed that he had privately told ministers a UK-EU trade deal might take 10 years to finalise, sparking criticism from some Members of Parliament.
He told them this was also the view of the EU’s other 27 member states.
Ministers have said a deal can be done within two years and reports say it appeared there had been “some failure of synchronisation” between Sir Ivan and the UK government.
Sir Ivan had been due to leave his post in November 2017.
Former Lib Dem leader, Nick Clegg, who once worked for Sir Ivan in Brussels, described his resignation as a “body blow to the government’s Brexit plans”.
Labour Member of Parliament, Hilary Benn, who chairs the Brexit select committee, said it had come at a “crucial” point and urged the government to “get its skates on” in finding a replacement.
“It couldn’t be a more difficult time to organise a handover,” he added.
Mr Benn said the permanent representative’s job was to convey the view of the UK to other member States, as well as “honestly and fearlessly reporting back” what those countries in turn said about the negotiations.
Prime Minister, Theresa May says she will trigger formal talks between the UK and the EU by the end of March, setting in place a two-year negotiation process
Sir Ivan is a veteran civil servant whose previous roles include private secretary to ex-chancellor Ken Clarke, principal private secretary to ex-Prime Minister, Tony Blair and Mr Cameron’s Europe adviser.
He was criticised in some quarters for “pessimism” over Brexit after his advice to ministers on the negotiating timescale was reported.
EU chairman, Arron Banks said Sir Ivan was “yet another of the establishment’s pro-EU old guard” and welcomed his resignation.