David Cameron makes claim as he responds to official inquiry into intelligence agencies’ actions before killing of fusilier

Internet companies are allowing their networks to be used to plot “murder and mayhem”, David Cameron has said in response to the official inquiry into the intelligence agencies’ actions before the killing of Lee Rigby.

He demanded that internet companies live up to their social responsibilities to report potential terror threats and said there was no reason for such firms to be willing to cooperate with state agencies over child abuse but not over combatting terrorism.

His comments to the House of Commons came after the parliamentary intelligence and security committee (ISC) concluded that the brutal murder of Rigby could have been prevented if an internet company had passed on an online exchange in which one of the killers expressed “in the most graphic terms” his intention to carry out an Islamist jihadi attack.

Praising the agencies as silent heros, he nevertheless admitted there had been errors by the agencies since both of Rigby’s murderers, Michael Adebolajo and Michael Adebowale, had been known to the security services for some time.

The 191-page report by the ISC says that if MI5 had had access to the exchange between one of the killers, Adebowale, and an extremist overseas, Adebowale would have become a top surveillance priority. By failing to alert the authorities, the company had, “however unintentionally”, provided a “safe haven for terrorists”.

The intelligence agencies, MI6 as well as MI5, meanwhile escaped with only light criticism despite the fact that they were monitoring both men. The intelligence committee, which is headed by the former foreign secretary Malcolm Rifkind, found that MI5, broadly responsible for domestic intelligence, had made errors but that the murder would have happened even if the errors had not.

Cameron announced an extra £130m over two years for intelligence agencies to deal with self-starter terrorists, as well as to fund the Prevent programme for tackling radicalisation and counter-terror policing. He disclosed that he would give the role of the security services commissioner a statutory basis, providing guidance on the treatment of terrorist suspects, including those held by counter-terror units overseas with which the UK co-operates.

The government already has a code on treatment of suspects but the current commissioner, Sir Mark Waller, will have a new power to report on the treatment of detainees.

Cameron said there was no possible justification for US internet providers not to inform agencies of terrorist activity since they already cancelled the accounts of suspected terrorists.

This summer, the government updated its legislation to require internet companies to cooperate with the state and report potential terrorist activity, but he said the level of cooperation was not satisfactory.

However, he admitted there was legal uncertainty about the duty of internet companies based in the US to cooperate with UK agencies due to conflicting laws in the US. The company that did not inform the agencies has not been named by the government.

In its report the committee found the response of MI6, responsible for overseas intelligence-gathering, in dealing with the other killer, Michael Adebolajo, to be inadequate. But overall there was little that the two agencies could have done to prevent the attack, the report says.

Rigby had been returning home on 22 May 2013 at the end of a shift at an army recruiting office at the Tower of London when he was hit by a car driven by Adebolajo in Woolwich, south London. The two killers then attacked the soldier with knives.

The committee inquiry was set up to investigate the role of the intelligence agencies, which had the two men under surveillance. The report says the two men appeared between them in seven different agency investigations but were for the most part regarded as low-level subjects of interest.

“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.

“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.”

The committee says that at any time MI5 is investigating several thousand individuals who are linked to Islamist activities in the UK.

Cameron has already appointed his national security adviser, Sir Nigel Sheinwald, as his special envoy on intelligence and law enforcement data-sharing to try to improve cooperation with US firms.

The former Labour foreign secretary Jack Straw said the leadership of some internet firms had a cultural problem and they were gripped by a “distorted libertarian ideology that allows them to feel they can detach themselves from the state”.

Cameron agreed with Straw’s description and said he planned to lead a debate with internet firms.

The prime minister said: “We must not accept that these communications are beyond the reach of the authorities or the internet companies themselves.

“We have taken action. We have passed legislation. And we will continue to do everything we can.

“We expect the internet companies to do all they can too. 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 it.”

Cameron did not announce any new legal powers beyond those to be included in the counter-terror bill to be published on Wednesday. It is likely the bill was framed with knowledge of the central conclusions of the ISC report.

Downing Street sources said Cameron did not expect to make any more progress on communication data laws this side of a general election. But he made clear his support for greater powers for the agencies, saying: “Are we prepared to have a means of communication using the internet which we do not have the means to intercept? My answer is no.”