Cabinet Secretary Mark Sedwill began an investigation, and in a meeting with Williamson today, May told the Defence Secretary he hadn't cooperated to "the same standard" as others.
"I put to you the latest information from the investigation, which provides compelling evidence suggesting your responsibility for the unauthorised disclosure," May wrote in her letter sacking Williamson. "No other credible version of events to explain this leak has been identified."
Opposition politicians have called for criminal proceedings against Williamson. A government spokesman said that while it wasn't up to the Government, as far as May was concerned the matter was now closed.
May's move to clamp down on unruly ministers comes the night before local elections, in which the Tories are expected to face a drubbing for their failure to deliver Brexit.
The meeting last week was convened to discuss the results of a six-month review of Britain's telecom supply chain, including Huawei's role in the rollout of so-called 5G networks. Britain is set to toughen the rules under which the Chinese firm operates in the country while stopping short of an outright ban, according to people familiar with the matter.
The issue has divided politicians, while the UK's allies are also watching closely. The US has warned countries not to include equipment from Huawei or other Chinese suppliers.
Huawei denies that its equipment is vulnerable to state espionage.
Williamson has previously tried to use confrontation with China as a way to burnish his credentials as Defence Secretary. In February, he threatened to deploy a warship to Asia on its maiden voyage, saying it would be a show of strength against China's territorial aspirations in the region.
A Chinese official accused Williamson of a return to a "Cold War mentality," while Chancellor of the Exchequer Philip Hammond had to cancel a trip to China in the aftermath.
Williamson is also known for telling reporters that Russia should "go away and shut up".
He denied he was responsible for the leak in his letter to May. It's rare for ministers to be fired, and more usual for them to accept an invitation to resign.
"I appreciate you offering me the option to resign, but to resign would have been to accept that I, my civil servants, my military advisers or my staff were responsible: this was not the case," he wrote.
May named Penny Mordaunt, who had been international development secretary, as Williamson's replacement.
A Royal Navy reservist and daughter of a paratrooper, Mordaunt was the first woman to serve as armed forces minister, and is now the first female Defence Secretary.
Mordaunt is pro-Brexit and seen as a potential leadership candidate. She made headlines in 2014 when she used a parliamentary speech on poultry welfare laced with innuendo to settle a bet with some Navy comrades.
She has argued that British foreign aid spending should be accountable to UK officials rather than international charities, a move that adds to her popularity among grassroots Conservatives.
Rory Stewart, another potential leader but one who has been loyal to May and backs her Brexit deal, replaces Mordaunt as international development secretary.
- Bloomberg
