Ivan Rogers came under fire for saying it could take 10 years for London to conclude deal with the bloc

File photo of UK's ambassador to the European Union Ivan Rogers. [Photo/China Daily]

UK's ambassador to the European Union resigned on Tuesday, adding uncertainty to the Brexit process less than three months before the country 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, stepped down as London prepares to invoke Article 50, which starts a two-year countdown to the United Kingdom leaving the European Union.

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

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 government spokeswoman said.

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

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

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

He also criticized the UK 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".

In probably the most damning passage, Rogers said that senior civil servants don't know what the priorities of Theresa May's government's are in the talks, which will set the course for UK's relations with Europe once it leaves the bloc.

"We do not yet know what the government will set as negotiating objectives for the UK's relationship with the EU after exit," he said.

An European diplomat said Rogers'resignation is not a surprise.

"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 percent voted for the UK to leave the EU.

Concerns over timing

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

Hilary Benn, who chairs Parliament's Brexit scrutiny committee, said 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.

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.