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.
At the last general election, Mr Johnson held the Orpington seat by a 19,453 majority.