An inquiry into how Muslim extremists were able to murder a British soldier on the streets of London listed a string of intelligence failures Tuesday but said that only information held by an Internet company might have stopped the attack.

The British parliament's intelligence and security committee (ISC) also warned that government programmes to prevent radicalisation were "not working", a particular concern as hundreds of Britons go to fight in Iraq and Syria.

Michael Adebolajo, 29, and Michael Adebowale, 23, are currently serving life in prison for running Lee Rigby over with a car and repeatedly knifing him outside his Woolwich barracks in May 2013.

The ISC report, the result of an 18-month inquiry, revealed the men featured in seven investigations by the security services going back to 2008.

But despite a string of mistakes and missed opportunities by those agencies, it concluded: "We do not consider that, given what the agencies knew at the time, they were in a position to prevent the murder of Fusilier Lee Rigby."

Instead, the committee criticised an unnamed Internet company for failing to flag up an online exchange in which Adebowale said he wanted to kill a soldier in retaliation for Britain's military interventions in Iraq and Afghanistan.

The December 2012 exchange with an individual thought to be an extremist with links to Al-Qaeda in the Arabian Peninsula (AQAP) was only discovered when the company carried out a review of Adebowale's accounts after the murder.

The report said companies had a duty to identify and report such conversations, saying: "This is the single issue which, had it been known at the time, might have enabled MI5 to prevent the attack."

The government is set to publish a new anti-terror law on Wednesday, and the ISC criticism will boost controversial efforts to increase the monitoring of online communications.

Responding to the report in the House of Commons, Prime Minister David Cameron announced another �130 million ($200 million, 165 million euros) for the security agencies over the next two years, and said Internet firms must act.

"Their networks are being used to plot murder and mayhem. It is their social responsibility to act on this. And we expect them to live up to it," he said.

However, Adebolajo's brother Jeremiah condemned any attempt to use the murder to tighten controls on social media.

"It is disingenuous to lead the public to believe that a 'lone wolf' attack, such as the one seen in May 2013, might have been prevented by closer monitoring of our online activities," he said in a statement.

- Concerns linked to Syria, Iraq -

The report catalogued the agencies' failures in detail, and warned of the implications for efforts to stop Britons joining Islamic State (IS) fighters in Iraq and Syria.

One of the most serious criticisms was MI6's failure to follow up on Adebolajo's arrest in Kenya in 2010 on suspicion of trying to join Shebab militants in Somalia.

The foreign spy agency's "apparent lack of interest" in the arrest was at odds with its responsibility to disrupt "jihadi tourism", the ISC said.

MI6 also failed to investigate his allegations of mistreatment in detention in Kenya and did not accord Adebolajo sufficient priority on his return home to Britain.

The parliamentary report also warned that "the government's counter-terrorism programmes are not working".

"Successfully diverting individuals from the radicalisation path is essential, yet 'Prevent' programmes have not been given sufficient priority," it said.

The report also criticised the "unacceptable" length of time it took to open an investigation on Adebowale, even accounting for the fact he was a low priority.

It said the authorities, used to dealing with terror networks, must adapt to the increasing threat from "self-starting terrorists".

And it warned MI5 does not have a strategy for dealing with suspects who appear on the edge of various investigations, and must take this "cumulative effect" more into account.