GCHQ (government communication headquarters) in Cheltenham

Just one in eight of the 1,700 warrants granting permission for intrusive surveillance to undercover agents last year was scrutinised by the watchdog charged with oversight of MI5, MI6 and GCHQ.

Sir Mark Waller, the intelligence services commissioner, said, in a rare public appearance on Tuesday, he was nevertheless “absolutely” certain that Britain’s spy agencies abided by the letter of the law. He drew pointed questions from a committee of MPs where the commissioner had been summoned to give evidence.

“I have a very good idea as to what the ethos of the [agencies] is,” Sir Mark, a former High Court and Court of Appeals judge, told the home affairs committee. “When I started the job I had a scepticism about the agencies . . . but when you get down there and see the care, you suddenly realise these people don’t want to break the law.”

The commissioner, who was appointed by the prime minister in January 2011, said he visited each agency just twice a year and described his scrutiny of the warrants granted for surveillance activities as a “dip sampling” approach.

Sir Mark said he typically looked at about 12 per cent of all warrants granted.

Keith Vaz, the committee’s chairman, said: “Some may feel that this isn’t particularly robust.”

As well as the intelligence services commissioner, some of the activities of MI5, MI6 and GCHQ are also overseen by the interception of communications commissioner. The intelligence and security committee of parliament, also appointed by the prime minister, has an overarching responsibility for monitoring the agencies’ work.

Speaking of the Snowden leaks last year – when a US government security contractor stole and leaked thousands of classified documents relating to sometimes controversial US and UK electronic surveillance activities– Sir Mark said on Tuesday his instant reaction was to fear he may have been duped by the services.

“I was absolutely terrified that all that I had thought for 18 months was now going to turn out to be utterly untrue,” he told the committee, adding that he “hared it” to GCHQ and met the agency’s second-in-command who reassured him that nothing was awry.

In one exchange, Mr Vaz appeared to question the depth of Sir Mark’s scrutiny, which he said appeared to amount to having “a chat”.

“It seems from your comment that what you did was have a discussion with them,” Mr Vaz said. “

Certainly,” Sir Mark replied, sparking a back-and-forth exchange:

“You heard what they had to say?”

“Certainly”

“Is that it?”

“Certainly”

“Just a discussion?”

“Certainly.”

Any suggestion that GCHQ was hiding some of its activities from his oversight was highly improbably, the commissioner said. “It would have to be a massive conspiracy.”

Sir Mark also rejected calls for a better resourced agency to scrutinise the intelligence services’ work.

The commissioner had initially declined an invitation to appear in front of the home affairs committee, which is conducting an inquiry into the UK’s counter terrorism policies.

The committee exercised its powers to compel Sir Mark to attend – the first time it has taken such a measure in this parliament.

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