British Prime Minister Boris Johnson's director of communications resigned on Wednesday, a move that suggested tension at the heart of ­government as Britain prepares to complete its Brexit journey out of the European Union.

Lee Cain has been a loyal ally of the prime minister, working for him since he was foreign minister until 2018 and staying by his side, sometimes without pay, when Johnson resigned over predecessor Theresa May's Brexit plans.

Cain is also close to Johnson's senior adviser, Dominic Cummings, after working with him in the Vote Leave campaign to quit the EU, a partnership that was reborn in Downing Street when the Conservatives won a big election victory in 2019. 

But sources have suggested there has been tension in No.10 Downing Street office since the government's communications strategy was criticized by Conservative lawmakers for being slow and confusing during the coronavirus crisis.

With Britain's official COVID-19 death toll now more than 50,000, the main opposition Labour Party said the public deserved better than the government "fighting like rats in a sack over who gets what job."

A Labour spokeswoman said it was "precisely this lack of focus and rank incompetence that has held Britain back."

Cain said in a statement that "after careful consideration I have this evening resigned as No.10 director of communications and will leave the post at the end of the year."

Johnson described Cain "as a true ally and friend," thanking him in a statement for his "extraordinary service."

The BBC said the prime minister's official spokesperson James Slack would take over as Johnson's director of communications at the end of 2020.