SOLDIER Lee Rigby's murder may have been prevented if an American web firm had flagged up a "graphic and emotive" online chat between one of his killers and an overseas extremist, a parliamentary watchdog has said.

In a 192-page report, the Intelligence and Security Committee labelled the unnamed internet company a "safe haven for terrorists" for failing to take action against suspected extremists after it emerged killer Michael Adebowale had promised to slay a soldier in an exchange sent six months before the attack.

The group of MPs, chaired by Conservative MP Sir Malcolm Rifkind, said it was "highly unlikely" the intelligence agencies would have seen the discussion, which came to light only after the barbaric murder near Woolwich Barracks, London, on May 22 last year, without the company's help.

The committee also concluded the three intelligence agencies - MI5, MI6 and GCHQ - could not have prevented the murder of Fusilier Rigby despite a litany of errors and missed opportunities in seven previous operations featuring Adebowale and his older accomplice Michael Adebolajo, both Muslim converts.

Human rights campaigners and civil liberties groups have raised concerns the committee has "spun the facts" to shift blame on net firms and away from the intelligence agencies.

But Fusilier Rigby's uncle Raymond Dutton, who lives in Greater Manchester, said: "I don't believe it could have been avoided. At the end of the day, the services are so stretched. I'm a firm believer Lee was in the wrong place at the wrong time.

"Perhaps learning from the report is what we can do with the information we have gleaned from this sad murder of my nephew."

Adebowale was in contact with an extremist now known to have links to al Qaeda in the Arabian Peninsula in late 2012, the report said. However this was not revealed until an unidentified third party notified GCHQ after the attack.

In the exchange, Adebowale expressed his desire to murder a soldier in retaliation for UK military action in Iraq and Afghanistan, although he had not developed a plan as to how he might carry out an attack - but the extremist advised him on different methods of murder, including using a knife.

It later emerged a number of online accounts owned by Adebowale were automatically disabled due to association with terrorists and terrorism but the web firm was unaware because it did not manually review such decisions.

Nor did the firm, which the committee refuses to name, notify law enforcement agencies.

Announcing £130 million in funding to improve the agencies' capability to combat "self-starting" terrorists, Prime Minister David Cameron attacked internet firms for failing to help tackle the threat.

"Terrorists are using the internet to communicate with each other and we must not accept that these communications are beyond the reach of the authorities or the internet companies themselves," he said. "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 that responsibility."

Sir Malcolm said the US firm "could have made a difference" by flagging the online exchange.

"This was highly significant. Had MI5 had access to this exchange at the time, Adebowale would have become a top priority," he said. "There is, then, a significant possibility that MI5 would have been able to prevent the attack.

"However, this company does not regard themselves as under any obligation to ensure they identify such threats, or to report them to the authorities,. We find this unacceptable. However unintentionally, they are providing a safe haven for terrorists."

Adebolajo and Adebowale are both serving a minimum of 45 years respectively.