Internet companies and social media sites have "a duty of care" to share information with authorities if they find a user plotting an act of terrorism, according to the family of murdered soldier Lee Rigby.

Step-Dad Ian Rigby said it was "a necessary evil" for social media giants like Facebook to pass on information in extreme cases if more deaths were to be prevented.

Watch: The family of Lee Rigby speak to GMB

One of the men that brutally killed Fusilier Rigby last May, Michael Adebowale, was found to have said he wanted to kill a soldier in an online chat with an overseas extremist months before the attack took place.

The Intelligence and Security Committee (ISC) said in a report released yesterday that intelligence agencies could not have prevented the murder, despite his killers featuring in a total of seven error-filled operations before the attack.

"The report doesn't do what it is supposed to do. It has brought a lot of memories back...and it doesn't give the correct information as to the reason to why the delay has happened," Mr Rigby said in a broadcast exclusive interview with Good Morning Britain.

Lee's sister Claire McClure said the report had given her someone to blame, but "was not sure if that was what I wanted".

While some blame should be levelled at MI5, she felt more let down by the internet companies.

Lee Rigby was mowed down by Michael Adebowale and Michael Adebolajo in a brutal attack in broad daylight near the Royal Artillery Barracks in Woolwich.

The two men said the attack was to revenge Muslims killed by western governments.

Both men were found guilty of murder and received full life sentences, with only Adebowale eligible for parole after 45 years.

David Cameron speaking to the House of Commons after the ISC report was published. Credit: PA

David Cameron said internet companies needed to do more to stop terrorists from using social media sites to plot "murder and mayhem" after the ISC report was published.

Rigby's family said the Prime Minister had attended Lee's funeral but other than that, they had heard nothing from Mr Cameron.

Many questions remained answered about Lee's death, Mr Rigby said, and a meeting with the PM would help the family find out what really happened to their son and brother.

Mr Rigby added: "He's the only one that has the full report and the only one that can use the answers that we want."