Theresa May has been dealt another Brexit-related blow as the UK's ambassador to EU quit the top diplomatic post on Tuesday (3 January). Sir Ivan Rogers' decision to resign as Britain's permanent representative to the EU comes just months before the prime minister plans to start separation talks with Brussels.

The dramatic move, first reported in The Financial Times and later confirmed to IBTimes UK by a source close to the Foreign Office, also comes after Rogers' warning that it could take up to 10 years for negotiations between the UK and EU to be completed.

The father-of-two read modern history at Balliol College, Oxford, before joining the civil service.

Rogers' career has included service as chief adviser to former Chancellor Ken Clarke, chief of staff to fellow Conservative Leon Brittan in the European Commission and principal private secretary to former Labour Prime Minister Tony Blair.

He served as European and global affairs advisor to David Cameron between 2011 and 2013 before the ex-Conservative premier promoted him to the UK's ambassador to the EU.

In the role, he was a major player in Britain's renegotiation with Brussels before the EU referendum in June, which saw the UK vote 52% to 48% to leave the bloc.

"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," said Arron Banks, the co-founder of the Leave EU campaign. "He has at least done the honourable thing in resigning."

Ivan Rogers
Britain's ambassador to the European Union Ivan Rogers is pictured leaving the EU Summit in Brussels on 28 June 2016Francois Lenoir/Reuters

May has promised to invoke Article 50, the mechanism to split from the EU, by the end of March 2017. Michel Barnier, the EU Commission's chief Brexit negotiator, has said he expects talks to be completed in October 2018 and a deal ratified in March 2019.

"The resignation of somebody as experienced as Sir Ivan Rogers is a body blow to the Government's Brexit plans," said Nick Clegg, the Liberal Democrat's EU spokesman and former deputy prime minister.

"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."