Gavin Williamson was snared as the Huawei leaker shortly after he admitted to an 11-minute phone call with the Telegraph journalist who broke the story.
Williamson told investigators working for Cabinet Secretary Sir Mark Sedwill that he spoke to Steven Swinford just hours after the National Security Council meeting.
However, he denied that he leaked any information from the secret meeting and said he only spoke about Brexit and Theresa May's impending departure from No 10.
After this admission, investigators turned their focus on him. He was forced to hand over his phone so that his calls, texts and emails could be scrutinised.
An investigator was even sent to interview the former Defence Secretary while he was on holiday with his wife Joanne and their two daughters in Scotland over Easter.
Sir Mark Sedwill's team interviewed all ministers in the frame for the leak, demanded they hand over their phones to monitor their calls, texts and WhatsApp messages and gave them a questionnaire asking to spell out their contact with journalists.
But they eventually concluded that Williamson was the culprit and he was sacked by Theresa May.
Mr Swinford, a former Sunday Times journalist who has been the Daily Telegraph's deputy political editor since 2015, has refused to reveal his source or discuss the nature of the call with Mr Williamson.
He wrote last week that his story was 'undeniably a matter of significant public interest'.
He wrote: 'It is a decision that has already led to bitter divides between ministers and MPs on all sides of the Commons, and has the potential to open a rift between Britain and its most significant ally - the United States'.
Gavin Williamson (left) was on holiday in Scotland with his wife Joanne Williamson and their children over Easter but was still grilled by a civil servant sent there from London by investigation chief Sir Mark Sedwill (right)
Sir Mark was charged with finding the source of the leak after the National Security Council meeting on April 23 where Mrs May refused to rule out letting Huawei help bring 5G to Britain - despite its links to China's spy network.
Defence Secretary Gavin Williamson's sacking by the Prime Minister follows days of political drama surrounding the leak of information from the UK's National Security Council.
Here is how the leak developed into a major Government inquiry:
- April 23 - A meeting of the UK's National Security Council (NSC), the country's top national security body, is held.
- April 24 - The Daily Telegraph newspaper reports that the Prime Minister has agreed to allow Chinese telecoms giant Huawei to help build Britain's new 5G network despite security concerns raised by Cabinet minsters at the meeting.
Labour demands an official investigation into the leak from the highly secretive council.
- April 25 - Dominic Grieve, chairman of the Parliamentary Intelligence and Security Committee, says the leak is 'deeply worrying'.
Home Secretary Sajid Javid says it is 'completely unacceptable' for any minister to release sensitive information and that it should 'absolutely be looked at'.
Gavin Williamson and Foreign Secretary Jeremy Hunt publicly deny they are responsible.
In a statement, Mr Williamson says neither he nor any of his team had 'divulged information from the National Security Council'.
- April 26 - An ultimatum is reportedly issued to ministers over the leak of the secret discussions.
Reports emerge that Cabinet Secretary Sir Mark Sedwill has demanded ministers in attendance at the NSC meeting confess or deny if they were behind the leak.
Downing Street refuses to say whether an inquiry is under way, despite calls for police to become involved.
Sources close to International Trade
