Sir Ivan Rogers, Britain's ambassador to the European Union (EU) since 2013, made the surprise decision to resign Tuesday, just a few weeks before Prime Minister Theresa May intends to begin formal negotiations regarding the terms of Britain's exit from the trading bloc.

In a note to staff Tuesday, Rogers did not explain his rationale for leaving. A government spokesperson told the BBC that he had taken the decision "a few months early" – he would otherwise have left in November – in order to "enable a successor to be appointed before the UK invokes Article 50 by the end of March." Article 50 is the section of the Lisbon Treaty that allows a country to leave the European Union. Invoking it would trigger formal exit negotiations, which according to the treaty must be concluded within two years barring a unanimous decision to extend the deadline. 

Reports have linked Rogers' sudden departure to a memo leaked to the BBC in December, in which the ambassador expressed the fear that a UK-EU trade deal could take a decade to finalize. Brexit boosters have argued that a deal could be finished within the time allotted by Article 50, based on the belief that Britain enjoys an enviable negotiating position. Pro-Brexit publications reacted with scathing coverage: "as usual," the Daily Mail wrote, Rogers "is spouting doom and gloom."

Skeptics doubt that a deal could be reached quickly, or that it would be particularly favorable, since the increasingly unpopular and economically vulnerable EU has a strong incentive to discourage other countries from jumping ship. (See also, Brexit: Would the Norwegian Model Work?)

Britain voted in a June referendum to leave the EU, taking pollsters by surprise and triggering the resignation of Conservative Prime Minister David Cameron, whom the party replaced as leader with then-Home Secretary May. (See also, Brexit Redux: How Pollsters Got the U.S. Election Wrong.)