A former head of MI5 believes the United States will seek a deal with whistleblower Edward Snowden to prevent him divulging more secret intelligence material.
Dame Eliza Manningham Buller said she was opposed to the publication of files leaked by the former CIA agent because newspapers could not know what damage it had done to counter-terrorism operations.
Mr Snowden, who embarrassed Washington and London by revealing the existence of mass surveillance programmes, is now in Russia which granted his temporary asylum after he fled the US.
Asked if he should face prosecution, Dame Manningham Buller told BBC Radio 4's Today: "I think what will happen actually is some kind of deal that he doesn't release any more but I really don't know."
The ex-security chief, who was guest editing the programme, said: "I do understand that there are people who think he has done a public service and who applaud him but I can't really be one of them because what neither the Guardian nor really anyone, including me, can judge is what damage he has done to counter-terrorism."
It was impossible for anyone other than the security services to know what terror plots had "gone dark" as a result of the information being made public or which might "not now be investigated, not now be thwarted", she said.
"My concern is the damage which I don't think anybody outside of the intelligence community can really detect or judge," she added - suggesting the thrust of the activities could have been published "without revealing the scale".
"I don't think those who have published can possibly work out what those consequences are because they don't have access to the information."
She told the programme: "The debate should not be what the capability of the state is, because the terrorist has that capability. But what should be authorised and who does the authorisation and what are the limits on it."
Tim Berners-Lee, the inventor of the internet, has defended the whistleblower - agreeing that he had "done us all a favour" by leaking the information.
During his own guest editing of the programme yesterday, he said figures such as Snowden should be seen as a "really important part of the system".