Internet companies face intense demands to monitor messages on behalf of the state for signs of terrorist intent after an official report into the death of Fusilier Lee Rigby said one of his killers wrote on a website – later named as Facebook – of his desire to slaughter a soldier, without the security services knowing.

The report said the authorities were never told that one of the killers, Michael Adebowale, wrote of his murderous intent six months before he and his accomplice, Michael Adebolajo, brutally attacked Rigby in May 2013 in a street near his military barracks and attempted to behead him.

Facebook had not spotted Adebowale’s message containing “graphic” threats, so the security services were not told. The report by the parliamentary intelligence and security committee (ISC) said if the message had been passed to MI5 it could have prevented the murder of the soldier.

The ISC chair, Sir Malcolm Rifkind, accused internet companies of providing a “safe haven” to terrorists but said a despite a string of failings by the security services, which had repeatedly monitored both men before the attack, there was nothing they could have done to prevent the murder of Rigby.

The report said that if the authorities had been given the message, “there is then a significant possibility that MI5 would have been able to prevent the attack”. The findings put internet companies and privacy firmly in the sights of lawmakers and the prime minister.

David Cameron vowed to take action but an internet rights group warned against co-opting companies and turning them into an arm of the surveillance state.

The ISC report also said the British government may have been complicit in ill-treatment suffered by Adebolajo during his detention in Kenya in 2010, where he complained of being beaten, and threatened with electrocution and rape after being arrested with other alleged extremists. It also criticised evidence from former MI6 chief Sir John Sawers, saying it was wrong and a document showing this had initially not been given to the committee by the intelligence service.

The government announce a new package of counter-terrorism measures on Wednesday , triggering a new round in the debate over the balance between security and privacy that has raged since the attack on the World Trade Center in 2001.

Rifkind said of Facebook: “This company does not regard themselves as under any obligation to ensure that they identify such threats, or to report them to the authorities. We find this unacceptable: however unintentionally, they are providing a safe haven for terrorists.”

In the Commons, Cameron ratcheted up the pressure: “Their networks are being used to plot murder and mayhem. It is their social responsibility to act on this and we expect them to live up to that responsibility.”

The ISC said in its report: “Whilst we note that progress has started to be made on this issue, with the Data Retention and Investigatory Powers Act 2014 (Drip) and the appointment of the special envoy on intelligence and law-enforcement data-sharing, the problem is acute. The prime minister, with the National Security Council, should prioritise this issue.”

Facebook had closed down some accounts by Adebowale and in a statement made after being named as the company in the report, 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.”

It had an automated program to stop message content deemed unacceptable – but the message discussing murdering a soldier was not picked up until after the killing.

Isabella Sankey, director of policy for the civil rights group Liberty, said: “The ISC shamefully spins the facts seeking to blame the communications companies for not doing the agencies’ work for them.”

Jim Killock, of Open Rights, which campaigns on issues such as online surveillance, censorship and privacy, said: “To pass the blame to internet companies is to use Fusilier Rigby’s murder to make cheap political points. It is quite extraordinary to demand that companies proactively monitor email content for suspicious material.”

The report said even if the security servicesfailings it identified had not occurred, their knowledge of the two extremists would not have led analysts to assess them as poised to strike.

Adebolajo, the more dominant of the two , had featured in five MI5 investigations and Adebowale in two, but none found evidence of an attack.

The ISC said MI5 made errors and was plagued by delays, but even if corrected none of this would have helped the security service to spot the level of danger posed by the attackers before they struck.

The committee said that at any time MI5 investigated several thousand individuals linked to militant Islamist activities in the UK.

The inquiry was set up to investigate the role of the intelligence agencies, which had the two men under surveillance. “There were errors in these operations, where processes were not followed, decisions not recorded, or delays encountered. However, we do not consider that any of these errors, taken individually, were significant enough to have made a difference,” the report says.

“Adebolajo was a high priority for MI5 during two operations: they put significant effort into investigating him and employed a broad range of intrusive techniques. None of these revealed any evidence of attack planning,” the committee says.

“By contrast, Michael Adebowale was never more than a low level SoI [subject of interest] and the agencies took appropriate action based on the rigorous threshold set down in law: they had not received any intelligence that Adebowale was planning an attack and, based on that evidence, more intrusive action would not have been justified.”

Rigby was returning to barracks in Woolwich, south London, on 22 May 2013 from an army recruiting office at the Tower of London when he was run over by a car driven by Adebolajo. The two killers then attacked the soldier with knives.

Ray Dutton, Rigby’s uncle, 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.”

The report also concluded that:

Given that there are 500 or more Britons fighting in Syria and Iraq, years of counter-terrorism policies in the UK had failed: “The scale of the problem indicates that the government’s counter-terrorism programmes are not working.”

The ISC refused to confirm or deny claims that MI5 had tried to recruit Adebolajo in Kenya as an informant. He was detained in Kenya in 2010 trying to join Islamist militants in neighbouring Somalia, and eventually came back to Britain. The ISC said: “To publish information in response to allegations that MI5 harassed Adebolajo or tried to recruit him as an agent would damage national security – irrespective of the substance of such allegations.”

MI6 – formally known as the Secret Intelligence Service – was strongly criticised for the way it dealt with the allegation that Adebolajo was ill treated in Kenya: “The committee is concerned by SIS’s approach on this occasion to allegations of mistreatment, which appears dismissive. Prejudging allegations in this way is completely inappropriate.” Parts of the report are redacted but the prime minister received the full version.

The Guardian understands senior figures in MI6 expressed anger at the criticisms in the report. One source familiar with the committee’s work said: “It is fair to say that the chaps across the river are not happy at all.”

The report’s final recommendation is redacted and reads: “Adebolajo’s allegations of mistreatment potentially related to a ***. It is essential that ministers are informed immediately of any allegations made against an overseas organisation for which any part of HMG [the government] bears responsibility and which is ***.”

Despite the reports findings, some members of the committee warned against any attempt by government to use it to justify greater surveillance powers and were concerned about the government’s handling of the report this week.

ISC member and Liberal Democrat MP Sir Ming Campbell said: “It is a remarkable coincidence, some might say, that the home secretary should have chosen to make public her further proposals on the eve of the publication of the ISC report. No doubt the purpose of doing so was to link her proposals to the committee’s conclusions. The committee never considered those proposals.”

ISC member Julian Lewis, a Tory MP, condemned leaks claiming the report would support greater powers on communications data. He said: “Nothing in our report is relevant to that argument”.

In Woolwich at the site of Rigby’s murder outside his barracks, a memorial was still in place. Enda Mcniffe, a 22-year-old student who has lived in Woolwich all his life, said the community now shared a real sense of fear, but that it had “grown stronger” after the attack. “From what I read at the time, there was information circulating about [the killers before the murder] and I thought it could have been acted on a bit more,” he said. “But it was going to happen, if not to poor Lee Rigby then to someone else.”

• Additional reporting by Nicholas Watt and Haroon Siddique