“It appears there are no other options but to step down from my position as Secretary of State for Wales,” Hart said in a Twitter resignation letter.
Earlier, Johnson fired Housing and Utilities Minister Michael Gove, who expressed the opinion that the head of government should leave his post.
On July 5, British Chancellor of the Exchequer Rishi Sunak and his counterpart, Health Minister Sajid Javid, left their posts.
After that, five more ministers left the government and called on Boris Johnson to resign.
Johnson, in turn, said that Cabinet members should not quit during a crisis.