Media playback is unsupported on your device Media caption Lee Rigby's sister and stepfather spoke to ITV's Good Morning Britain

It is "unfair" to ask firms such as Facebook to check all website postings for possible terrorist content, a former MI6 director has said.

Richard Barratt said it would be "almost impossible" to require firms to pass on terror-related online activity.

His comments came after an MPs' report said an unnamed company, now known to be Facebook, could have done more to prevent Fusilier Lee Rigby's death.

Facebook said it did not allow terrorist content and aimed to stop it.

Some members of Fusilier Rigby's family have said they hold Facebook partly responsible for his murder.

Michael Adebolajo and Michael Adebowale killed the soldier outside Woolwich barracks in south-east London in May 2013.

Drummer Rigby's sister, Sara McClure, reportedly told the Sun the company had her brother's "blood on their hands".

According to the newspaper, the soldier's stepfather, Ian Rigby, said: "Facebook failed us all when they failed to alert our authorities."

Media playback is unsupported on your device Media caption Lee Rigby's uncle: "I can't believe... Lee's murder could have been stopped"

He also said the MPs' report "doesn't do what it's supposed to do" and "left a lot of questions".

Speaking to ITV's Good Morning Britain he said Prime Minister David Cameron was "probably the only one that could get the answers that we want".

Mr Cameron has said he will meet the Rigby family. "Whatever meetings they want, they can ask for and they will get," he said.

On Tuesday, the prime minister said big internet companies had a "social responsibility" to act on terrorist material posted online.

His comments came after a report published by the Intelligence and Security Committee (ISC) into the murder found one of the killers had discussed plans to kill a soldier "in the most graphic and emotive manner" five months before the attack.

The ISC report said social media websites could become a potential "safe haven for terrorists" if they failed to act on threats such as this.

The UK authorities became aware of the exchange only in June 2013, a month after Fusilier Rigby was murdered.

Media playback is unsupported on your device Media caption Richard Barrett, former Director of Global Counter Terrorism Operations for MI6. on tracing terrorists on social media

Committee chairman Sir Malcolm Rifkind said that if the security services had had access to the exchange there was a "significant possibility that MI5 would have been able to prevent the attack".

The parliamentary inquiry also identified a number of errors, serious delays and potential missed opportunities in security operations but concluded Fusilier Rigby's death could not have been prevented.

It highlighted that the UK's security agencies say they face "considerable difficulty" accessing content from Facebook and five other US tech firms: Apple, Google, Microsoft, Twitter and Yahoo.

Legal basis

Speaking to the BBC's Radio 4 Today programme, Richard Barrett, former director of global counter-terrorism operations for MI6, said Facebook would need legislative guidance before being asked to gather terror-related activity.

"I think it's unfair to ask companies to make that decision. I think there has to be a legal basis for them so they know what they are to do and what not do to," he said.

But he also said that getting an international agreement on that legal basis would be "almost impossible".

"If people want to get around the restrictions that are placed on their communications by companies like Facebook they probably can quite easily," he said.

In the UK "there are about 25 million users of Facebook" and "possibly 125 million posts a day", so passing terror-related activity would be an "enormous job," he added.

Image copyright Met Police Image caption Michael Adebolajo, left, and Michael Adebowale

'Terrorist content'

The ISC's report identified a "substantial" online exchange during December 2012 between Adebowale and a foreign-based extremist - referred to as Foxtrot - who had links to the Yemen-based terror group AQAP, but was not known to UK agencies at the time.

After the murder of Fusilier Rigby an unidentified third party provided a transcript of the conversation to GCHQ.

The information was also said to have revealed that Facebook had disabled seven of Adebowale's accounts ahead of the killing, five of which had been flagged for links with terrorism.

This had been the result of an automated process, according to GCHQ, and no person at the company ever manually reviewed the contents of the accounts or passed on the material for the authorities to check.

Media playback is unsupported on your device Media caption Nick Clegg said "alarms should go off" if material of concern is posted online

GCHQ noted that the account which used the phrase "Let's kill a soldier" was not one of those closed by Facebook's software.

A spokesperson for Facebook said: "Like everyone else, we were horrified by the vicious murder of Fusilier Lee Rigby."

"We don't comment on individual cases but Facebook's policies are clear, we do not allow terrorist content on the site and take steps to prevent people from using our service for these purposes."

New counter-terror laws to increase powers for police and security services are set to be unveiled by Home Secretary Theresa May later.