Intelligence agencies could not prevent the murder of soldier Lee Rigby despite his killers featuring in a total of seven error-filled operations before the attack, a parliamentary watchdog has found.

In a long-awaited 192-page report, the Intelligence and Security Committee (ISC) said it did not consider any of the opportunities missed by MI5, MI6 or GCHQ in the run-up to Fusilier's Rigby's barbaric murder by Michael Adebolajo and Michael Adebowale significant enough to have affected the outcome.

But the ISC, chaired by Conservative MP Sir Malcom Rifkind, labelled an unnamed internet company a "safe haven for terrorists" after an online exchange was discovered - after the attack - between Adebowale and an overseas extremist in which he reveals he intended to murder a soldier.

The committee said that if MI5 had known about the exchange, there was a chance it would have been able to prevent the attack - but due to lack of action by the internet company and powers available to the security services, it is "highly unlikely" the agencies could have accessed it on their own.

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 May last year 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 mo st detailed report the group of MPs has ever published.

The inquiry discovered t he two men appeared, between them, in seven different agency investigations, for the most part as low-level subjects of interest.

The report said the ISC "discovered a number of errors" in these operations where processes were not followed, decisions were not recorded or delays were encountered.

But the group of MPs adds: "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."

Adebolajo was a high priority for MI5 during two operations. The agencies put "significant effort" into investigating him and employed a broad range of intrusive techniques.

None of these revealed any evidence of attack planning.

Adebowale was never more than a low-level subject of interest, the report said.

But after the attack, an online exchange between Adebowale and an overseas extremist in December 2012 emerged.

Adebowale expressed his intent to murder a soldier in the "most graphic and emotive manner", the report said.

It said: "This was the exchange - not seen until after the attack - between Adebowale and an individual overseas in December 2012.

"In this exchange, Adebowale told FOXTROT that he intended to murder a soldier.

"Had MI5 had access to this exchange, their investigation into Adebowale would have become a top priority.

"It is difficult to speculate on the outcome but there is a significant possibility that MI5 would then have been able to prevent the attack."

The report says it is highly unlikely that the agencies could have obtained the intelligence and instead attacks the internet company that hosted the exchange.

It said: "The party which could have made a difference was the company on whose platform the exchange took place.

"However, this company does not appear to regard itself as under any obligation to ensure that its systems identify such exchanges, or to take action or notify the authorities when its communications services appear to be used by terrorists.

"There is therefore a risk that, however unintentionally, it provides a safe haven for terrorists to communicate within."

The report also criticises MI6's "apparent lack of interest" in Adebolajo's arrest in Kenya as "deeply unsatisfactory".