The serving boss of the UK’s home security agency told the Today programme 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 preparing legislation on the powers for carrying out electronic surveillance.
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”.
But Shami Chakrabarti, director of human rights group Liberty, said while she agreed that existing surveillance legislation was “inadequate”, she was concerned about “any attempt to seek a blank cheque from the British public for unlimited surveillance”.
Meanwhile, a highly critical report from David Anderson QC, the independent reviewer of terrorism legislation, warns forthcoming separate legislation to ban extremism could provoke a damaging backlash in Muslim communities.
The lawyer said in his annual report that if the new laws were too widely drawn, they could play into the hands of people trying to encourage extremism and terrorism.
MI5 boss Mr Parker also told the BBC:
- The terrorism threat is the “most serious threat Britain faces in security terms”
- Six alleged terror plots have been foiled in the past 12 months, which Mr Parker said was the highest number he could recall in his 32-year career “certainly the highest number since 9/11”
- 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.”
Read more: http://www.bbc.co.uk/news/uk-34276525