Terror threat at its highest in 30 years, says UK's spy chief Andrew Parker
The head of MI5 said that terrorist plotting against Britain was at its most intense for three decades as he backed new powers to monitor communications.
Andrew Parker said new technologies were posing greater challenges for his agency and argued that firms such as Facebook and Twitter had a responsibility to share information.
But he stressed that MI5 was not interested in "browsing through the private lives" of the public and should work within a transparent legal framework.
Giving the first live broadcast interview by any security service chief on BBC Radio 4's Today programme, Mr Parker said the police and security services had foiled six terror plots in the past six months and warned the threat was still growing.
He added: "That is the highest number I can recall in my 32-year career, certainly the highest number since 9/11. It represents a threat which is continuing to grow, largely because of the situation in Syria and how that affects our security."
He also backed the Government's Investigatory Powers Bill - dubbed a snooper's charter by critics - and said: "Because of the way terrorists operate, it is necessary that if we are to find and stop the people who mean us harm, MI5 and others need to be able to navigate the internet to find terrorist communication.
"The important thing to say is that we focus on the people who mean us harm. We are not about browsing through the private lives of citizens of this country."
The MI5 head also referred to heavy criticism of Facebook over its failure to disclose a key conversation involving one of Fusilier Lee Rigby's killers.
"In that case, the Intelligence and Security Committee concluded that had that happened it might have made a difference to the outcome," he said. "If there is something that concerns terrorism or child exploitation or some other appalling crime, why would the company not come forward?"
Asked why the security services failed to prevent the killing of Fusilier Rigby or anticipate the actions of extremists such as Jihadi John, Mr Parker said officials had to make choices. "There cannot be a guarantee that we can find and stop everything - that would be impossible," he added.
But Shami Chakrabarti, the director of the civil rights pressure group Liberty, said Mr Parker's backing of the Government's legislation was an attempt to legalise questionable spying activity.
She told BBC Radio 4's World At One: "It is revealing that he alluded to the fact that there needs to be a law to adequately describe the sorts of things MI5 does.
"It suggests to me that Edward Snowden (who leaked classified US intelligence) was right - that vast amounts of private information is being compromised, and that it is happening outside the law without public consent or parliamentary debate. I am concerned about any attempt to seek a blank cheque from the British public for unlimited surveillance."