Edward Snowden, the US whistleblower who leaked thousands of secret NSA files to the Guardian, has exposed formidable technologies that go beyond anything imagined by George Orwell, the newspaper's editor-in-chief said yesterday.

Defending the Guardian's reporting of the Snowden files in the wake of criticism made by the head of MI5 on Tuesday, Alan Rusbridger also said there was a contrast between the hostile official British reaction to the disclosures and the response in the United States, where Barack Obama has called for a debate on the extent of state surveillance. Speaking on The World at One on BBC Radio 4, Rusbridger said: "They [the authorities] have information they couldn't dream of harvesting 10 years ago and they will always want more. There has to be a balance.

"There have been instances in the last few months where people have gone through metadata in order to find out reporters' sources … These technologies are formidable. They are beyond what Orwell could have imagined."

He added: "I would be very surprised if the current oversight methods really understood and knew about some of the things we have been describing as a result of the Snowden revelations."

Rusbridger was speaking after Downing Street had offered strong support to Andrew Parker, the MI5 director-general, on the morning after his Tuesday night speech. Parker had described leaks as a "gift" to potential terrorists, although he did not mention Snowden by name.

When asked whether Cameron agreed with Parker that the leak of the Snowden files had been a gift to potential terrorists, at the morning lobby briefing, the No 10 spokesman said: "I would happily point you to all parts of the director-general's speech. It was an excellent speech."

When asked specifically, whether the prime minister's endorsement covered Parker's claim that making public GCHQ's techniques provided "the gift they [terrorists] need to evade us", the spokesman said: "Including that."

Parker had defended the work of GCHQ and warned of the dangers of publishing documents which reveal some of its systems, remarks appeared to be aimed at the whistleblower who gave thousands of intelligence files to the Guardian that revealed surveillance programmes carried out by GCHQ and its US counterpart, the National Security Agency (NSA).

In his Tuesday night address to the Royal United Services Institute, Parker had said: "Such information hands the advantage to the terrorists.

"It is the gift they need to evade us and strike at will. Unfashionable as it might seem, that is why we must keep secrets secret, and why not doing so causes such harm."

But in response, Rusbridger challenged Parker's claim that the leaking and publication of the NSA files posed a risk to national security.

He told Radio 4: "I don't think some of this will come as a great surprise to terrorists," and noted that Obama had chosen to establish a review panel that includes Peter Swire, a former White House aide, to examine the surveillance methods used by the country's intelligence agencies.

In Washington, Ron Wyden, a Democratic member of the Senate intelligence committee who has led congressional attempts to rein in the NSA, also dismissed Parker's claims."There's a real difference between secret operations and secret law," he said. "I take a back seat to no member of Congress when it comes to protecting the secrecy of what's called sources and methods. Secret operations have to be sacrosanct. But that's not what we're talking about, we're talking about the policies that undergird this surveillance debate."

Clive Stafford Smith, the legal director of death row campaigners Reprieve, dismissed Parker's claim that the Snowden files had helped terrorists.

He said: "It must be said that there is nothing in the Guardian's revelations that has been shown to help terrorists in the least, and Mr Parker did not give a single concrete example to demonstrate this."

Eric King, head of research at Privacy International, said: "Andrew Parker is right: the UK isn't East Germany. While the Stasi had files on one in three East Germans, the communications of almost everybody in the UK are being intercepted and stored as part of GCHQ's Tempora programme. Our security agencies' continued insistence that they are not prying, while every week new mass surveillance programmes are being revealed, is offensive to the public's intelligence."