Jo Johnson, younger brother of Prime Minister Boris Johnson, is quitting as a Conservative MP and minister, saying he is “torn between family loyalty and the national interest”.
The business minister and MP for Orpington, tweeted that there was an “unresolvable tension” in his role.
Mr Johnson voted Remain in the 2016 EU membership referendum.
He resigned as a minister under Theresa May last year in opposition to the Brexit deal she reached with Brussels.
But he re-entered government during the summer, after Conservative Party members elected his brother as leader.
Following Mr Johnson’s resignation, former cabinet minister David Gauke, an MP who was removed from the Conservative Party after backing an anti-no-deal Brexit bill in Parliament this week, tweeted: “Lots of MPs have had to wrestle with conflicting loyalties in recent weeks. None more so than Jo. This is a big loss to Parliament, the government and the Conservative Party.”
Labour MP Wes Streeting tweeted: “Whatever our disagreements, I very much respect Jo’s integrity and always appreciated his decency and courtesy in our discussions on higher education policy. The Conservative Party is losing too many One Nation Tories.”
At the last general election, Mr Johnson held the Orpington seat by a is 19,461 majority.