The government has been accused of trying to spin a long-awaited report on the murder of Lee Rigby to bolster its case for broader snooping powers for the security services.
The Intelligence and Security Committee (ISC) report, published on Tuesday, said MI5, MI6 and GCHQ could not have prevented the murder of Lee Rigby - but said an unnamed American internet provider could have given "decisive" help by reporting Michael Adebowale, one of the killers, having an online conversation graphically describing a potential attack five months before it happend.
The ISC said this failure meant the terrorists had "a safe haven" online for communicating.
Julian Lewis, a Tory MP who sits on the ISC - accused the government of leaking parts of it to intensify the debate around the legal powers spies have to monitor suspects' web activity.
The report coincides with Theresa May's comments talking up the threat terrorists pose this week, reigniting debate about the Snooper's Charter - a bill that would massively enhance the security services' legal rights to to monitor peoples' internet activity. But It is languishing in parliament after the Lib Dems withdrew their support for it.
Labour MP George Howarth, who sits on the ISC, criticised government attempts to spin its conclusions almost immediately after its publication.
He said it did not make case for any additional communications powers and the government would be "wrong" to use report to try to do that.
Jim Killock, executive director of digital rights activists Open Rights Group, said: "The government should not use the appalling murder of Fusilier Rigby as an excuse to justify the further surveillance and monitoring of the entire UK population.
"To pass the blame to internet companies is to use Fusilier Rigby’s murder to make cheap political points.
"The committee is particularly misleading when it implies that US companies do not co-operate, and it is quite extraordinary to demand that companies proactively monitor email content for suspicious material. Internet companies cannot and must not become an arm of the surveillance state."
ISC Chair Sir Malcolm Rifkind insisted the release of the report was not timed to coincide with the introduction of a major new counter-terrorism bill in the Commons on Wednesday.
According to the report, Adebowale had not come up with a definite plan of how to carry out an attack when he spoke online to the other extremist, dubbed "Foxtrot".
Writing in a piece published in The Telegraph to coincide with the report, Rifkind said the fact Adebowale's conversation was not brought to the security services' attention was "striking and deeply concerning".
"Until this problem is resolved, our intelligence agencies will not be able to find some of the people communicating in cyberspace who are plotting terrorist atrocities," he wrote.
"These people will be discovered only after they have carried out their atrocities in the real world, once it is too late.
"Internet companies like Apple, Facebook, Google, Microsoft, Twitter, Yahoo and many others need to play their part in alerting authorities to people who may be terrorists."
The ISC report largely exonerated MI5, MI6 and GCHQ of any blame though it noted Adebowale and Adebolajo both featured in seven previous MI5 operations - though only as low level subjects of interest.
Though mistakes were made in these operations, they did not make a different to the fate of Lee Rigby, the ISC said.
"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,” it said.
In response to the report, David Cameron announced £130 million in funding to improve the agencies' capability to combat "self-starting" terrorists - also known as lone wolf attackers.
He told the Commons: "There are lessons to be learned and things that need to change."