Why UK ambassador to the EU resigned?

The British ambassador to the European Union Ivan Rogers has unexpectedly resigned just three months before Theresa May intends to trigger Article 50 and initiate exit talks, 10 Downing Street confirmed. (Business Insider)

Why did he resign?

BBC: Sir Ivan Rogers endured an uncomfortable time at the last EU summit in December after his confidential advice to the government about Brexit potentially taking 10 years was leaked to the BBC. No ambassador relishes “becoming the story” in that way. There’s a sense in Brussels that he may have been seen as a pessimist by cabinet Brexiteers because it fell to him to convey the hostility and scepticism with which other governments view Brexit.

Independent: (…) clear tensions with No 10 were laid bare in mid-December when it was reported that he had warned the Government it could take 10 years to complete a post-Brexit trade deal with the EU. He was afterwards lambasted by the pro-Brexit Tory MP Dominic Raab, who accused the diplomat of being “pessimistic”.

The Guardian: Rogers angered Eurosceptics in December when it emerged he had told ministers it could take 10 years to negotiate a free-trade deal with the EU. Downing Street insisted at the time that the ambassador had been communicating the views of some European leaders, rather than giving his own assessment.

Daily Mail: ‘Now let’s appoint an EU ambassador who believes in Brexit!’ Britain’s ‘pessimistic’ man in Brussels quits after facing major backlash over his ‘gloomy’ warnings about EU divorce.