Video report by ITV News Security Editor Rohit Kachroo

The director general of MI5 has called on tech companies to create methods which would allow the security services to access the secret, encrypted messages of people suspected of plotting terrorist attacks in the UK.

Speaking to ITV, Sir Andrew Parker says while the real world is regulated and policed, he finds it "mystifying" the same does not apply to cyberspace, calling it "a wild west, unregulated [and] inaccessible to authorities."

Some messaging apps use end-to-end encryption, which means the content of messages can only be read by the sender and recipient, and cannot be intercepted by a third party - such as security services.

Sir Andrew told ITV News Security Editor Rohit Kachroo: "I say to the [tech] companies: Can you please use the brilliant technologists you’ve got to answer this question, which is: Can you provide end-to-end encryption but on an exceptional basis – exceptional basis – where there is a legal warrant and a compelling case to do it, provide access to stop the most serious forms of harm happening?"

Sir Andrew, who is standing down as director general of MI5 in April, has been speaking to ITV News and the Tonight programme as we were given unprecedented access inside the agency, which works on counter-terrorism and counter-espionage. Over three months, our cameras were allowed to film inside the heart of the security service for the first time, for an edition of Tonight to be broadcast on Thursday.

In a wide-ranging interview, Sir Andrew also said that MI5 'cannot monitor closely' every person who is plotting terror, following attacks carried out in 2017 by people who had been known to MI5.

I [have] usually have said to ministers, successive home secretaries over the years: if and when there’s another terrorist attack, the very high likelihood is that it will be done by somebody who appears in our records somehow, because we have worked really really hard at trying to understand which people in this country are across this landscape of extremism. And there are thousands of them and we cannot – cannot – monitor closely what all those people are doing all the time. MI5 director general, Sir Andrew Parker

During six months in 2017, five attacks happened - Westminster Bridge, Manchester, London Bridge, Finsbury Park and Parsons Green. Asked if he felt he wasn't in control of the security situation at the time, Sir Andrew said: "Well we’re not in control of it ever, are we?"

"To be in control would mean that somehow we could manage this whole landscape and stop everything. We can’t. We can’t do that."

Andrew Parker said he had a 'sinking feeling' when he first heard about the Manchester terror attack, which killed 22 people Credit: ITV News

On what he felt when he first heard about the Manchester attack which killed 22 people, Sir Andrew said: "I think I’d be lying if I didn’t say I had a sinking feeling. Of course, you’re speaking to me as the director general of MI5 but, you know, I’ve got a family. I’ve got kids. And, yeah, of course – it’s the worst of the worst situation."

He also said most of the UK population should not be concerned that MI5 is spying on them in any way.

"We are only interested in people who mean us harm. We’re interested in terrorists and spies, and that’s where we focus our collection", he said.

"We do not approach our work by population level monitoring – looking for, you know, signs of: 'Out of this 65 million people, who should we, you know, look a bit more closely at?'"

"We do not do that."

The director general of MI5 is the only name in the entire organisation whose name can be made public. After thirty years, Sir Andrew Parker had to give up his anonymity when he got the top job, surprising many friends and family who didn't know he had been a spy for all that time.

Just close family and one or two friends [knew] where I was working. But everybody else, you know, didn’t know that I was working here, thought I was working for the Government in some sort of role. A day came when my face and job description and new appointment was on the news – nearly seven years ago. Let's say it was at least interesting for people who know me. MI5 director general, Sir Andrew Parker