Advances in technology are allowing terrorists to communicate “out of the reach of authorities”, the head of MI5, Andrew Parker has told the BBC.
The serving boss of the UK's home security agency told Today it was becoming more difficult to obtain online information.
He said internet companies had an “ethical responsibility” to alert agencies to potential threats.
But MI5 was not about “browsing the lives” of the public, he added.
Ministers are currently preparing legislation on the powers for carrying out electronic surveillance.
But Mr Parker, in the first live interview by a serving MI5 boss, said what should be included in new legislation was a matter “for parliament to decide”.
“It is completely for ministers to propose, and parliament to decide. It's a fundamental point about what MI5 is. It's for us to follow what's set by parliament, and that's what we do.”
Mr Parker also told the BBC:
- The terrorism threat is the “most serious threat Britain faces in security terms”
- MI5 had to “make choices” about where to put resources, and make sure they were “focused where the sharpest threat is”
- On the killers of Fusilier Lee Rigby: “There cannot be a guarantee that we will find and stop everything. That's not possible. We can't monitor them all the time.”
- He rejected the suggestion that security service tactics can lead to radicalisation saying it was “completely untrue”
- He paid tribute to the people who work at MI5 and their work “which so often goes unrecognised”
He said online data encryption was creating a situation where the police and intelligence agencies “can no longer obtain under proper legal warrant the communication of people they believe to be terrorists”.
It was a “very serious” issue, he said, adding: “It's in nobody's interests that terrorists should be able to plot and communicate out of the reach of authorities.”
This live interview by a serving head of MI5 is unprecedented. There are several reasons for it. One is that the threat of terrorism is high at the moment because of the conflict in Iraq and Syria, and the concern over jihadists returning to the UK.
But the security service is also concerned about technological change making it harder to access communications. New legislation on that is expected in the next few months, and it's likely MI5 think this is the time to speak out to set the tone for that debate.
If you talk to the people at MI5, they would say it's not so much about new powers, but maintaining their existing capabilities against changing technology. Of course critics are sceptical and say it's all part of a campaign for new powers.
Another issue for MI5 is access to communications held abroad – for example by technology companies in Silicon Valley.
Andrew Parker said there was an “ethical responsibility” for companies to come forward with information on terrorist or criminal activities. Technology companies don't necessarily see that as their job, they see it as the spies' job. There are tensions there, that have been growing.
Mr Parker said the shape of the threat had changed “because of the internet and the way terrorists use social media”.
He said they were using secure and encrypted apps and the internet to “broadcast their message and incite terrorism among people who live here”.
“Most of the people who try to become involved in terrorism in this country are born and brought up here, come through our education system”, he said. But they had “decided the country of their birth is their enemy”.
The MI5 chief said there was a question of the ethical responsibility of companies like Facebook and Twitter to alert the authorities regarding information about terrorism, child sex exploitation and other criminal activity.
“I think there is a real question about companies who hold that information under what arrangements they should come forward and report it,” he said.
“If its something that concerns terrorism, child sex exploitation or some other form of crime, why would a company not come forward?”
He also rejected the suggestion that MI5 tactics led to the radicalisation of targets and played down fears about extremists entering Europe among the thousands of refugees from Syria.