In response, Williamson strenuously denied any involvement and said, "I am confident that a thorough and formal inquiry would have vindicated my position".
The discussion took place in a top secret government meeting called the National Security Council on 23 April.
Meetings often involve sharing of secret intelligence and ministers are bound by the Official Secrets Act.
Metropolitan Police Commissioner Cressida Dick indicated that it would be for ministers to trigger an inquiry. May as Prime Minister.
In her letter on sacking Williamson, May, for her part, underscored that she considers "the matter closed", in an apparent indication that she would like to avoid a police investigation. Lord Augustine O'Donnell, who was cabinet secretary under three Prime Ministers from 2005 to 2011, told the BBC that leaks from the NSC were "incredibly serious, a complete outrage".
Labour's deputy leader, Tom Watson, said the government was trying to shut down legitimate questions about possible criminal activity by a minister before a police inquiry had been launched.
"A kangaroo court with a summary execution".
He went on to ask the Prime Minister why she was ignoring the advice of two of the UK's closest intelligence allies and putting in danger the 70 year intelligence sharing relationship.
Kanter plays on despite separating shoulder more
Kanter originally suffered the injury in the deciding fifth game of the Blazers' first-round series with Oklahoma City. Denver's Nikola Jokic was Game 3′s Ironman, tallying 33 points, 18 rebounds and 14 assists in a whopping 65 minutes.
"Unfortunately, as we have seen all too often recently, it is easier to throw unsubstantiated allegations at the civil servant advising the prime minister, than address the substance of her decision".
In a letter to Williamson, May said that the probe "provides compelling evidence suggesting your responsibility for the unauthorized disclosure".
Theresa May appointed Williamson as Defence Minister back in 2016 in a decision that raised eyebrows of her colleagues and the public, given that the appointee had no military background. Of course you have the whole Chinese State Security thing, but they also donated a bunch of phones to the RSPCA so that they could take better pictures of adoptable critters. Aw.
While there's no doubt the critical information leaked from the National Security Council, Mr. Williamson is adamant his department is not responsible for them.
Mr Williamson revealed that he rejected an offer from Mrs May to resign rather than be sacked, saying that this would have sent a signal that he accepted that either he or his team was guilty.
The investigation caused May to "lose confidence" in Williamson, and resulted in her asking him to leave the government, said 10 Downing Street in an official statement.
"Within the last week, Williamson sought to blame officials for the leak itself and then accused the Cabinet Secretary of waging a personal vendetta against him".
That information is said to have come from a leak of the UK's National Security Council.
But in relegating Mr Williamson to the backbenches in such a humiliating manner, the Prime Minister could be storing up problems for herself further down the line.