British lawmakers say two Islamic extremists who murdered a soldier in a London street had been under scrutiny by the intelligence services, and one had expressed his intention of killing a soldier in an online exchange months before the attack.

Parliament's Intelligence and Security Committee says that if British spies had known of Michael Adebowale's declaration, "there is a significant possibility" they could have prevented the murder.

But the online exchange was only discovered after Adebowale and Michael Adebolajo killed Fusilier Lee Rigby in May 2013.

The lawmakers concluded in a report published Tuesday that spy agencies had made mistakes, but that with the knowledge they had at the time they "were not in a position to prevent the murder of Fusilier Rigby."