LONDON _ Jo Johnson's decision to quit his own brother's Cabinet and not stand as a Conservative member of Parliament is a knife to the heart for the prime minister from his own family.
It's also the most vivid example of how Boris Johnson is splitting his Conservative Party.
"In recent weeks I've been torn between family loyalty and the national interest," Jo Johnson said in the tweet announcing his decision. "It's an unresolvable tension."
The suggestion that his own brother no longer trusts him to act in the national interest is a heavy blow to Johnson, especially as opposition parties have been using the question of the prime minister's integrity as their justification for not agreeing to a general election.
They fear he could go back on his word and seek a no-deal Brexit while Parliament is suspended for an election campaign.
Jo Johnson first resigned over Brexit a year ago, saying the choice facing the country _ between former Prime Minister Theresa May's divorce deal with Brussels and a no-deal Brexit _ was one between "vassalage and chaos." He called for another referendum to resolve the issue.
Then when his older brother ran for the leadership in the summer, he joined the campaign, apparently having accepted that Britain had to leave the European Union. He took the view that own electoral district of Orpington, on the edge of southeast London, would be likely to fall to the Brexit Party unless Britain left, according to a person familiar with his thinking.
Privately, he also took the view that Boris Johnson had no interest in a no-deal Brexit, and would seek a deal with the EU and then force hard-line Brexiteers in the party to vote for it. His resignation suggests that view has changed.
This week, 21 Conservative MPs were expelled from the party for voting to block a no-deal Brexit.
