British Prime Minister Theresa May fired her defence secretary, Gavin Williamson, on Wednesday, saying an investigation suggested he was to blame for leaking discussions about Chinese telecoms company Huawei from her National Security Council.
Last week it emerged that Britain would allow Huawei a restricted role in building parts of its 5G network, seeking a middle way in a bitter dispute between the United States and China over the next generation of communications technology.
The NSC is a forum in which senior cabinet ministers discuss top secret national security information, and the leak, first reported in national newspapers, sparked anger in parliament.
May wrote to Williamson that an investigation into the leaks had provided "compelling evidence suggesting your responsibility for the unauthorised disclosure."
"No other, credible version of events to explain this leak has been identified," she added.
Williamson had repeatedly denied that he was responsible for the leak. A BBC correspondent said she spoke to him after his firing was made public and he continued to deny blame.
May appointed international development minister Penny Mordaunt to succeed Williamson as defence secretary, and named prisons minister Rory Stewart to Mordaunt's former role.
Huawei, the world's biggest producer of telecoms equipment, is under intense scrutiny after the United States told allies not to use its technology because of fears it could be a vehicle for Chinese spying. Huawei has categorically denied this.
"It is vital that I have full confidence in the members of my Cabinet and of the National Security Council," May wrote in her letter to Williamson, which was released by her office.
"The gravity of this issue alone, and its ramifications for the operation of the NSC and the UK's national interest, warrants the serious steps we have taken, and an equally serious response."