Gavin Williamson has been sacked as defence secretary following an inquiry into a leak from a top-level National Security Council meeting.
Downing Street said the Prime Minister had "lost confidence in his ability to serve" and Penny Mordaunt will take on the role.
The inquiry followed reports over a plan to allow Huawei limited access to help build the UK's new 5G network.
Theresa May said there was "compelling evidence" that he had been behind the leak of secret discussions about the Chinese technology firm Huawei - which has led America to threaten intelligence links with the UK.
Mr Williamson has told Sky News he wasn't responsible and that he may be the victim of a feud with the government's top civil servant.
Sky's political editor Beth Rigby reports.