Jo Johnson, the younger brother of Prime Minister Boris Johnson, is resigning as an MP and minister, saying he is “torn between family loyalty and the national interest”.
The business minister and Tory MP for Orpington, south-east London, cited an “unresolvable tension” in his role.
Mr. Johnson voted Remain in the 2016 EU membership referendum, while his brother co-led the Leave campaign.
BBC political editor Laura Kuenssberg said it was “unbelievable timing”.
Mr. Johnson’s resignation follows the removal of the Tory whip from 21 MPs this week for supporting moves to prevent a no-deal Brexit.