Soldier Lee Rigby's murder might have been prevented if a US web firm had flagged a "graphic and emotive" online exchange between one of his killers and an overseas extremist, a parliamentary watchdog has said.

In a long-awaited 192-page report, the Intelligence and Security Committee (ISC) labelled the unnamed in ternet company a "safe haven for terrorists" for failing to take action against suspected extremists after it emerged that killer Michael Adebowale had vowed to kill a soldier in an online chat six months before the attack.

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

The committee concluded that 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 operations featuring Adebowale and his older accomplice Michael Adebolajo.

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

In the exchange, he 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 advises him on different methods including using a knife.

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

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

Addressing a press conference at parliament, Sir Malcolm said: "The one issue which we have learned of which, in our view, could have been decisive only came to light after the attack.

"This was an online exchange in December 2012 between Adebowale and an extremist overseas, in which Adebowale expressed his intent to murder a soldier in the most graphic and emotive manner.

"This was highly significant. Had MI5 had access to this exchange at the time, Adebowale would have become a top priority.

"There is, then, a significant possibility that MI5 would have been able to prevent the attack."

Sir Malcom said the US firm "could have made a difference".

"However, this company does not regard themselves as under any obligation to ensure that they identify such threats, or to report them to the authorities," he said.

"We find this unacceptable - however unintentionally, they are providing a safe haven for terrorists."

Muslim converts Adebolajo and Adebowale ran down Fusilier Rigby, who was dressed in a Help For Heroes hoodie, in a Vauxhall Tigra near Woolwich Barracks, in south east London, before savagely attacking the defenceless soldier as he lay in the road.

The ISC inspected hundreds of highly-classified documents and questioned ministers, the heads of the three agencies and senior officers from the Metropolitan Police for its inquiry - described as the most detailed report the group of MPs has ever published.

The fanatics appeared, between them, in seven different agency investigations, the ISC said, which contained "a number of errors" including processes not being followed, decisions not being recorded, or delays.

But the group of MPs added: "We do not consider that any of these errors, taken individually, were significant enough to have made a difference.

"We have also considered whether, taken together, these errors may have affected the outcome.

"We have concluded that, given what the agencies knew at the time, they were not in a position to prevent the murder of Fusilier Rigby."

Fusilier Rigby's uncle Raymond Dutton said the report confirmed his belief that his nephew's murder could not have been prevented.

"Mistakes have been made possibly, perhaps we could have done a little bit more," he told BBC News.

"But in my heart of hearts I can't believe that - even with further evidence - Lee's murder could have been stopped. Everyone in Britain at that time was in this cocoon of safety on our streets. That was smashed by these two murderers for their religious gain."