BBC: Boris Johnson quits to add to pressure on May over Brexit
Boris Johnson has resigned as Foreign Secretary amid a growing political crisis over the UK's Brexit strategy.
He is the second senior cabinet minister to quit within hours following Brexit Secretary David Davis's exit, the BBC said.
His departure came shortly before Theresa May began addressing Parliament about her new Brexit plan, which has angered many Conservative MPs.
She said she did not agree with the two ex-ministers about "the best way to honour" the result of the 2016 vote.
The BBC's political editor Laura Kuenssberg said Mr Johnson's exit had turned an "embarrassing and difficult situation for the PM into potentially a full-blown crisis."
She said he was not any ordinary cabinet minister but was the "face" of the Leave campaign during the 2016 referendum and his departure would fuel speculation about a leadership challenge.
She said she had been told by a source that either Theresa May "dumps" the plan signed off at Chequers or "another minister will go, then another, then another, then another".