Laws used by Britain's spy agencies to justify mass surveillance and interception techniques must be reviewed to ensure they have kept pace with "incredible changes" in communications, one of the country's foremost intelligence experts has said.

Lord King, the former Conservative defence secretary who also chaired the cross-party intelligence and security committee (ISC) when it was established, added that the current committee under Sir Malcolm Rifkind had come out too quickly to back GCHQ following revelations about industrial-scale snooping.

King said the public needed to be completely reassured that "huge amounts of interceptions" undertaken by GCHQ had been "properly authorised under the normal standard arrangements". But he also suggested the laws giving them those powers could be out of date and need to be looked at again.

The principal piece of legislation, the Regulation of Investigatory Powers Act (Ripa), has been in force since 2000. "Back in 2001 people were not using email in the way they are now," said King. "As an MP at that time, I hardly received any emails at all. Now they receive hundreds.

"It is incredible to think how things have changed in the cyber world. Now you have the whole world of Twitter and Facebook, and all the other paraphernalia. Legislation has to keep up to date with all these things and the way people use them. I think it is most important that all the legislation in this area is under regular review."

Ripa has been used by British intelligence to provide legal authority for the Tempora programme, which gives GCHQ the ability to tap into vast amounts of data carried by undersea internet cables. In theory, GCHQ can store 21 petabytes of information a day – equivalent to sending all the information in all the books in the British Library 192 times every 24 hours. Privacy campaigners argue that Ripa was never intended to issue warrants that would justify such large-scale trawls.

Speaking to the Guardian, King said the ISC had a vital role working as a bulwark against the intelligence services overreaching themselves. The peer chaired the watchdog for seven years between 1994 and 2001 and is the longest serving head of the committee.

King said the decision by the Rifkind-led committee to swiftly endorse the work of GCHQ was "unfortunate" because the endorsement came while new disclosures from files leaked by the whistleblower Edward Snowden were still being published by the Guardian and other newspapers around the world.

"I think their response was pretty quick," said King, a former Northern Ireland secretary. "It came at a time when revelations were still coming out. It is very important the ISC maintains public confidence as a scrutiny committee."

King argued that the ISC should be chaired by a member of the opposition. Rifkind is a former Tory foreign secretary – the cabinet post that has ultimate responsibility for GCHQ.

"This is no criticism of Malcolm Rifkind," said King. "It's about giving the ISC credibility and I think this gives the committee more credibility."

He also said that the ISC might benefit from having independent experts seconded to help them and said the committee must have investigators who have intimate knowledge of the intelligence agencies.

"It is absolutely vital the ISC has an independent investigator, a poacher turned gamekeeper. We had a man who was an ex-intelligence officer who knew how they operated."

Without a specialist of this kind, the committee might find it hard to know where to look and what questions to ask, he added.

Separately, in an article for the Guardian, Sir David Omand, a former director of GCHQ, said he believed Britain's intelligence agencies would welcome "deeper but more informed oversight, not least to protect their reputation".

Omand, the UK's first security and intelligence coordinator reporting directly to the prime minister, said this was a significant time for the ISC, which has new powers to demand information from the agencies. He added that the committee now had to show that it could understand the issues that advanced intercept technology posed and must be given expert help to do that.

However, he also criticised Snowden, the former contractor at America's National Security Agency who leaked thousands of highly classified files to the Guardian. Snowden has been given temporary refuge in Russia.

Omand claimed there was only a thin public interest argument for publishing details of programmes such as Tempora and Prism, which has given the NSA the ability to trawl through the servers of mobile phone companies.

"I know the public does not want to make the internet a safe space for those who wish to harm us," Omand said. "It is not the existence and power of systems like Prism and the links from NSA and GCHQ to the internet companies that is the problem: that capability is what it takes to do the job society has asked them to do in an internet age.

"The real debate we should be having … is about what privacy in a cyber-connected world can realistically mean given the volumes of personal data we hand over to the private sector in return for our everyday convenience and the continued need for warranted access for security and law enforcement."

Omand said GCHQ's behaviour should not be verified by whistleblowers such as Snowden. He says the Chinese and Russians may already have gained access to some of the source material taken from the NSA. "Not even the KGB in its 1950s heyday could have dreamed of being in a position to do so much damage to western intelligence," he argued. "It will be like watching a slow motion car crash as lines of intelligence dry up, intelligence officers are exposed and cyber attackers learn to avoid our defences."