Leaked details of security surveillance programmes by whistleblower Edward Snowden have been the "most catastrophic loss to British intelligence ever", a senior Whitehall security expert has said.
Sir David Omand, the former head of GCHQ who was once security adviser to Number 10, said the leak of tens of thousands of files by the former US intelligence operative eclipsed the Cambridge spy ring, which saw five university students recruited as Soviet spies.
His remarks follow MI5 head Andrew Parker's warning that exposing the ''reach and limits'' of listening post GCHQ causes enormous damage and hands the advantage to terrorists.
Sir David told the Times that British officials assumed the information released by Mr Snowden was being analysed by Russian and Chinese spy agencies.
"You have to distinguish between the original whistleblowing intent to get a debate going, which is a responsible thing to do, and the stealing of 58,000 top-secret British security documents and who knows how many American documents, which is seriously, seriously damaging," he said.
"The assumption the experts are working on is that all that information or almost all of it will now be in the hands of Moscow and Beijing. It's the most catastrophic loss to British intelligence ever, much worse than Burgess and Maclean in the 1950s."
Donald Duart Maclean and Guy Burgess were among five people who passed information to the Soviet Union during the Second World War and at least into the early 1950s.
Snowden, who is in Russia, leaked information to the Guardian in May that revealed mass surveillance programmes such as the US National Security Agency (NSA) run Prism and the GCHQ-operated Tempora.
Under the £1 billion Tempora operation, Cheltenham-based GCHQ is understood to have secretly accessed fibre-optic cables carrying huge amounts of internet and communications data and shared the information with the NSA.
Yesterday, the first picture of Snowden since he accepted asylum in Moscow was published. He was pictured receiving an award for "integrity in intelligence" from four prominent American whistleblowers.