Home affairs committee asks home secretary whether she has been given proof by MI5 and MI6 to support their rhetoric

The home secretary, Theresa May, faced criticism from MPs on Monday for failing to provide evidence to support the "melodramatic soundbites" of Britain's spy agencies claiming that revelations from the whistleblower Edward Snowden had damaged the UK's national security.

During a 45-minute grilling by the home affairs select committee, May was repeatedly asked whether she had evidence to back up "highly emotional statements" made by the heads of MI5, Andrew Parker, and MI6, Sir John Sawers. They have said stories published by the Guardian were a "gift for terrorists".

Keith Vaz, the committee chair, asked five times whether the home secretary had been given proof to support the agencies' rhetoric. He asked: "These statements are made, but no evidence is put forward. Do you have any evidence today?"

May said she would not "sit here and talk about these things", but confirmed she had been in discussions with MI5 and was "clear in my own mind" that the stories about the work of Britain's surveillance headquarters GCHQ had damaged national security.

She added: "I am appalled at the fact that leaked information is published which could put at risk the lives of men and women who themselves put their lives at risk for this country."

Vaz replied: "Being appalled is not evidence."

When asked about the specific claims of the agency chiefs that Britain's enemies were "rubbing their hands with glee", May said: "I don't tend to use phrases like that."

Vaz said his committee was unanimous in believing that Parker should appear before them as they prepare a report on Britain's counter-terrorism strategy. The committee's invitation to Parker and the national security adviser, Sir Kim Darroch, to address them was blocked by May and the prime minister last week.

Two weeks ago the editor-in-chief of the Guardian, Alan Rusbridger, appeared before the MPs and defended the publication of stories based on the files leaked by Snowden. He said he could not "think of a story in recent times that has ricocheted around the world like this has and which has been more broadly debated in parliaments, in courts and amongst NGOs".

Rusbridger argued that news organisations that had published stories from the Snowden files had performed a public service, and highlighted the weakness of the scrutiny of agencies such as GCHQ and the NSA.

"It's self-evident," he said. "If the president of the US calls a review of everything to do with this and that information only came to light via newspapers, then newspapers have done something oversight failed to do."

He also quoted senior officials from the UK and the US who he said "have told me personally that there has been no damage. A member of the Senate intelligence committee said to us: 'I have been incredibly impressed by what you have done … I have seen nothing that you have done that has caused damage."

Over the last six months the Guardian has published a series of stories about the mass surveillance techniques of GCHQ and its US counterpart, the NSA.

Two of the most significant programmes uncovered in the Snowden files were Prism, run by the NSA, and Tempora, which was set up by GCHQ. Between them, they allow the agencies to harvest, store and analyse data about millions of phone calls, emails and search-engine queries.

In front of the committee on Monday, May conceded that a debate had started, though she said discussions about the balance between security and surveillance were not new. She said: "It is right that in a democracy we should have an ongoing debate about these issues to make sure we get the balance right."

Michael Ellis, a Conservative member of the committee, asked May why the head of MI5 "feels able to make melodramatic soundbites … and his colleagues made soundbites to get across their points" without providing evidence.

May said the agency chiefs should report primarily to parliament's intelligence and security committee (ISC), which takes a lead on scrutiny of the agencies and has recently been given new powers.

Vaz told the home secretary: "What you have given us today, and what we have heard so far, is only second-hand information. Mr Parker and Sir John are making statements in open session and nobody knows what the follow-up is."

He said his committee could not give a fair and balanced report on counter-terrorism without direct access to the agency bosses. Though he did not criticise the ISC, he pointed out the committee's limitations. He said heads of the intelligence agencies were appointed by the prime minister, as was the head of the ISC, Sir Malcolm Rifkind.

"Everyone is appointed by the prime minister," Vaz told May. "They are asking questions of each other, and giving answers to each other … That is exactly why we need to see them [the agency heads]. But you don't want us to see them at all."