LONDON (Reuters) - British Prime Minister Boris Johnson’s younger brother, Jo, has resigned as a junior minister and said he would also step down as a lawmaker, citing a conflict between family loyalty and the national interest.

FILE PHOTO: Britain's Minister of State for Business, Energy and Industrial Strategy Department and Education Department Jo Johnson is seen outside Downing Street in London, Britain, September 4, 2019. REUTERS/Hannah McKay/File Photo

His surprise resignation comes days after the prime minister expelled 21 Conservative lawmakers from the party for failing to back his Brexit strategy, including Winston Churchill’s grandson and a former finance minister.

Since taking office in July, Boris Johnson has tried to corral the Conservative Party, which is deeply divided over Brexit, behind his strategy of leaving the European Union on Oct. 31, with or without a deal.

That approach has shattered traditional party loyalties and caused him to lose both his parliamentary majority and control of the Brexit process, pushing Britain deeper into a national crisis ahead of the exit deadline and intensifying uncertainty over what path the country will take.

Jo Johnson, 47, who voted ‘remain’ in 2016, has previously expressed backing for a second referendum on whether Britain should leave the EU.

He nevertheless accepted a job as a junior minister in the business and education departments when his brother became prime minister, but quit that post on Thursday with what was seen as a parting shot at his brother.

“It’s been an honor to represent Orpington for 9 years & to serve as a minister under three PMs (Prime Ministers),” Jo Johnson said on Twitter.

“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 (Member of Parliament) & Minister,” he said.

The prime minister’s spokesman issued a statement thanking Jo Johnson for his service and adding: “The PM, as both a politician and brother, understands this will not have been an easy matter for Jo.”

He had been an MP for the Orpington constituency in Kent, south east England, since 2010, serving in several ministerial roles.

Boris Johnson’s “do or die” Brexit strategy, and his hardline approach to enforcing it, is causing a realignment in the ruling party, with several senior, more pro-European members announcing they will not seek re-election.

The Johnson family itself is notoriously split over Brexit with his sister Rachel having represented different parties opposed to Brexit, and his father Stanley being a committed europhile and former European Commission official.

The opposition Labour Party seized on the resignation, saying it underlined a lack of trust in the prime minister.

“Boris Johnson poses such a threat that even his own brother doesn’t trust him,” said Labour education spokeswoman Angela Rayner.

It is not the first time Jo Johnson, a former Financial Times journalist and a much less flamboyant character than his brother, has caught the government by surprise with a resignation notice.

Last year, he quit then-prime minister Theresa May’s government, calling in a withering critique for another referendum to avoid her Brexit plans unleashing Britain’s greatest crisis since World War Two.