British Prime Minister Theresa May has sacked her Defence Secretary for leaking top-secret information about Chinese mobile giant Huawei.

Ms May ordered Gavin Williamson to step down on Thursday (Australian time) after an investigation into leaks from a National Security Council meeting about the telecoms firm found him responsible for the “unauthorised disclosure of information”.

Announcing the decision, a Downing Street spokesperson said Ms May had “lost confidence in his ability to serve in the role of Defence Secretary”.

Downing Street later confirmed the appointment of Penny Mordaunt, the Minister for Women and Equalities, as the new Defence Secretary, making her the first ever female to take over the top job.

In a letter to Mr Williamson, Ms May said he cannot be trusted following the inquiry last week which found “compelling evidence” he leaked details about a meeting to allow Huawei to participate in some aspects of the future 5G mobile phone network.

Ms May said that “no other credible version of events” could explain the leak. However, Mr Williamson “strenuously” denied that he was the source. 

“I am sorry that you feel recent leaks from the National Security Council originated in my department … I am confident that a thorough and formal inquiry would have vindicated my position,” he wrote in a letter to Ms May.

Last week, London’s Daily Telegraph reported that Ms May agreed to allow Huawei to help build “non-core” elements of the country’s 5G network, such as antennas.

It said it had obtained details of security council meetings about Huawei and that several ministers, including Williamson, had opposed letting Huawei work on Britain’s 5G network.

The UK government insists no decision has been made about Huawei.

The United States has been lobbying allies to exclude Huawei from all 5G networks, noting that the Chinese government can force the company to give it backdoor access to data on its networks.

The security council includes senior ministers, who receive briefings from top military and intelligence officials, and its meetings are considered highly sensitive.

The leak comes amid a Brexit-fuelled breakdown in government discipline.

With Ms May weakened by her failure to take Britain out of the European Union, multiple ministers are positioning themselves to try to replace her, partly by cultivating positive press coverage.

-with AAP