Internet firms need to do more to meet their "social responsibility" to act to prevent their networks being used for terrorist purposes, the Prime Minister has said.
It comes after a report by the Intelligence and Security Committee labelled a US web firm a "safe haven for terrorists" after an online exchange by one of Lee Rigby's killers was discovered in which he reveals he intended to murder a soldier.
Mr Cameron said: "The Committee is clear and I agree that they have serious concerns about the approach of a number of communication service providers based overseas."
He added: "We will continue to do everything we can. But crucially 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 that responsibility."
Prime Minister David Cameron has announced an extra £130 million will be given to security and intelligence agencies.
Mr Cameron said the money would be used to "enhance our ability to monitor and disrupt these self-starting terrorists".
It comes in the wake of a report by the Intelligence and Security Committee into actions by agencies in the lead up to soldier Lee Rigby's death.
Prime Minister David Cameron has said lessons can be learned from a report into actions taken by intelligence agencies in the lead up to soldier Lee Rigby's murder.
Mr Cameron said: "There are lessons to be learned and things that need to change."
The report, by the Intelligence and Security Committee, concluded intelligence agencies could not have prevented the murder of Rigby despite his killers featuring in a total of seven error-filled operations before the attack.
The Intelligence and Security Committee raised concerns about the lack of monitoring by communication service providers and warned it left the British public open to "a higher level of threat".
The Committee said none of the providers it approached as part of its investigations into agency operations before the death of soldier Lee Rigby monitored and reviewed suspicious content on their systems.
Also none of the providers regarded themselves as compelled to comply with UK warrants.
"This is of very serious concern: the capability of the agencies to access the communications of their targets is essential to their ability to detect and prevent terrorist threats in the UK," the report said.
It said the UK Government was taking action on the issues but warned: "Until it is resolved the British public are exposed to a higher level of threat."
The Intelligence and Security Committee was asked whether the public will learn whether MI5 attempted recruit Michael Adebolajo in Kenya.
Rifkind - whether MI5 attempted to recruit Adebolajo not in the public report but was discussed and included in report that went to PM
Committee chair Sir Malcolm Rifkind said that information was not in the public report.
MI5 will neither confirm nor deny the suggestion as it would be damaging to national security, the committee said.
Sir Malcolm Rifkind said the US-based internet company which might have prevented the murder of Lee Rigby only provided the information to GCHQ on the condition of confidentiality and so it will not be named.
The Intelligence and Security Committee labelled an unnamed internet company a "safe haven for terrorists" after an online exchange was discovered - after the attack - between Michael Adebowale and an overseas extremist in which he reveals he intended to murder a soldier.
The committee said that if MI5 had known about the exchange, there was a chance it would have been able to prevent the attack - but due to lack of action by the internet company and powers available to the security services, it is "highly unlikely" the agencies could have accessed it on their own.
ITV News UK Editor Rohit Kachroo tweeted:
Given what security agencies knew at the time of the attack "they were not in a position to prevent the murder of Fusilier Rigby"
But Rigby's killers were on the radar. Between them, they appeared in seven different investigations
Investigation uncovered an online exchange between Adebowale and a foreign extremist in which he "expressed his intent to murder a soldier"
MI5 largely cleared. Criticism for American internet companies though, for "providing a safe haven for terrorists"
Lessons are important here - so-called 'self starter' attack like Woolwich considered to pose an increasing threat to UK national security
Intelligence agencies could not have prevented the murder of soldier Lee Rigby despite his killers featuring in a total of seven error-filled operations before the attack, the Intelligence and Security Committee has said in a report.
ITV News reporter Martha Fairlie reports: