"No other, credible version of events to explain this leak has been identified", she added.
Mr Williamson's abrupt firing as head of the Ministry of Defence sent shockwaves around Westminster on Wednesday after Theresa May said the inquiry had found "compelling evidence" suggesting he was to blame.
She also faced calls from Labour for the police to be involved, with the party's deputy leader Tom Watson arguing "It is not for the Executive to decide whether a crime has been committed".
Senior people from the National Cyber Security Council refused to comment on the leak when The Register asked them about it last week at their annual Cyber UK shindig in Glasgow.
Shortly after the news broke, Williamson released a statement where he "emphatically" denied any involvement in the leak and said that a formal inquiry "would have vindicated my position".
Williamson was offered the chance to resign.
In this photo from April 8, 2019, Britain's Defense Secretary Gavin Williamson (L) and Britain's International Development Secretary and Minister for Women and Equalities Penny Mordaunt arrive iat Downing Street in central London.
"That's why I think the logical extension of what the Prime Minister has alleged in her letter is - a criminal act has taken place and the police need to examine the facts".
EC directs to file FIR against Babul Supriyo
During 2018 panchayat polls, Derek O'Brien tweeted the number of deaths in previous elections to show Trinamool's rule was better. On Monday morning, a scuffle was reported between villagers and police officials near polling station in Asansol , West Bengal .
Williamson was sacked late Wednesday by May, who said she had seen "compelling evidence" that he was behind media reports that the government had agreed - against the advice of the United States - to let Huawei participate in some aspects of Britain's new 5G wireless communications network.
He said: "Apart from the extraordinary leak which I won't go into, the decision that is being discussed in many parts of the world is the possibility that we will be nesting a dragon in the critical national infrastructure of the United Kingdom by allowing Huawei to build the Cyber network that will power the UK".
Huawei has rejected allegations that it is too close to the Chinese government.
Mr Williamson blamed a vendetta by Sir Mark, who is also the Cabinet Secretary, for his sacking, The Times reports.
Williamson was one of the first to do so, calling it "completely unacceptable".
The 42-year-old was a trusted ally of the prime minister.
As a former defence secretary, Williamson is entitled to the normal severance settlement enjoyed by any cabinet minister, No 10 said, which is three months of his annual salary of £67,505.