Facebook has been named as the internet company which failed to pass on crucial information that could have stopped the murder of the British soldier who was decapitated on a high street.
Information about one of Fusilier Lee Rigby's killers could have enabled the British security services to prevent the murder in London.
Michael Adebowale used the social media site to communicate with another extremist - codenamed Foxtrot - in which he discussed plans to kill a soldier five months before the killing in Woolwich, a report by the Parliamentary Intelligence and Security Committee concluded.
British Prime Minister David Cameron used a statement in the Commons following the publication of the report to warn that internet companies' websites "are being used to plot murder and mayhem".
He said that it is "their social responsibility" and "moral duty" to act to prevent any potential attacks.
Fusilier Rigby (25) was run over and then butchered by Adebowale and Michael Adebolajo outside Woolwich barracks in south-east London on May 22, 2013.
The report by the committee concluded that Adebowale had used the internet site - now understood to be Facebook - to express "his desire to murder a soldier in the most explicit and emotive manner".
The internet firm in question had not been aware of the specific exchange relating to the killing of a soldier, the report said.
But, crucially, it concluded that the internet firm "had previously closed some of Adebowale's accounts because their automated system deemed them to be associated with terrorism". Despite that, the website did not review those accounts or pass "any information" to the authorities.
The report found that Adebowale's message about killing a soldier was "highly significant". If the security services had been made aware of it, they may have been able to stoop the murder, the report concluded.
"If Adebowale's exchange with Foxtrot had been seen by MI5 at the time, then we believe that the investigation would have increased to Priority 1, unlocking all the extra resources this would have entailed. This is the single issue which - had it been known at the time - might have enabled MI5 to prevent the attack."
The report said that there is "considerable difficulty" in accessing online communications on sites belonging to US-based internet firms such as Apple, Facebook, Google, Microsoft, Twitter and Yahoo.
A number of the companies do not co-operate with the UK security services because they claim they "need to protect users' privacy", it added. (© Daily Telegraph, London)