"In recent weeks I've been torn between family loyalty and the national interest," Jo Johnson tweeted

British Prime Minister Boris Johnson was dealt a fresh blow on Thursday when his brother Jo quit the government, saying he could no longer reconcile "family loyalty and national interest".

Jo Johnson had campaigned strongly against Britain's exit from the European Union in 2016, a position that put him at odds with his older and more famous brother Boris.

But he took a job in his brother's government as universities and science minister, a position he had held previously.

"It's been an honour to represent Orpington (a London suburb) for nine years & to serve as a minister under three prime ministers," Jo Johnson tweeted.

It's been an honour to represent Orpington for 9 years & to serve as a minister under three PMs. In recent weeks I've been torn between family loyalty and the national interest - it's an unresolvable tension & time for others to take on my roles as MP & Minister. #overandout - Jo Johnson (@JoJohnsonUK) September 5, 2019

"In recent weeks I've been torn between family loyalty and the national interest -- it's an unresolvable tension & time for others to take on my roles as MP & minister."

Like many families in Britain, the Johnsons were deeply divided over Brexit -- a third sibling, Rachel, and their father Stanley also wanted to stay in the EU.

Stanley worked for the European Commission in the 1970s and then served as a Conservative MEP, while Rachel ran unsuccessfully for the European Parliament in elections this year for the anti-Brexit Change UK party.

The resignation of Jo Johnson, who like his brother Boris is a former journalist, comes after 22 MPs left the governing Conservative party this week.

One MP defected to the pro-European Liberal Democrats and 21 were expelled for voting against the prime minister's Brexit strategy.

His also resignation comes the day after MPs voted to legislate to stop Boris Johnson taking Britain out of the European Union without a divorce deal on October 31.

The opposition Labour party seized on his departure.

Deputy leader Tom Watson tweeted: "Once again, the people who trust Boris Johnson least are the ones who know him best."

Pollster Joe Twyman tweeted: "It's going to be a hell of a Christmas lunch in the Johnson household".

BBC journalist David Cornock quipped: "A rare case of a politician resigning to spend less time with his family".