Prime Minister Theresa May's office says May has "lost confidence" in Williamson.
The government continues to insist that there is no need to involve the police, no need for a criminal investigation, no need for a trial of Mr Williamson.
Mr Williamson denies he is the culprit, and says getting the police involved would clear his name.
She stood in as Secretary of State for International Development in 2017, following Priti Patel's resignation over a number of unauthorized meetings with Israeli politicians.
The disclosure of details of the meeting, within hours of it taking place, outraged security officials and prompted ministers to write to May demanding an inquiry.
Foreign Secretary Jeremy Hunt, commenting on Mr Williamson's departure as he arrived in Addis Ababa, Ethiopia, said: "On a personal level, I'm very sorry about what happened for Gavin's sake, but given the gravity of the situation, there was no other alternative outcome".
Kelly Clarkson Performs At The 2019 Billboard Music Awards!!
We also got a taste of Ariana Grande's "7 Rings", but it was merely a pre-recorded performance from her Sweetener World Tour. Their performance will mark their very first award show performance in 10 years and fans are extremely excited about that.
The Prime Minister's sacking letter also said Williamson did not fully cooperate with an investigation into the leaks - which he has reportedly "sworn on his kids lives" he is not behind.
Mr Williamson said he was "massively comfortable" with the prospect of a police investigation into the leak, and told Sky News that it would result in the "nicest apology" from the Prime Minister.
Penny Mordaunt, the worldwide development secretary, has been appointed as Williamson's replacement.
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.
Labour deputy leader Tom Watson requested a statement from Theresa May on the findings of the leak inquiry, but it was unclear whether the Prime Minister would respond in person or send another minister.
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 concerns it could be a vehicle for Chinese spying.
Police are unlikely to open a criminal investigation unless the case is referred to them by the Government, the country's most senior officer has indicated.