Security experts from Cheltenham's GCHQ oversaw the destruction of hard drives linked to a Guardian newspaper journalist whose partner was subsequently arrested by British authorities, it has emerged.
Bosses at the paper say they have been put under increasing pressure from "shadowy Whitehall figures" to surrender material they have been working on which is linked to the security leaks by former US intelligence worker Edward Snowden over the past two months.
It comes after the Guardian lifted the lid on the extent of information sharing between British and American security agencies – with reporter Glenn Greenwald leading the way in asking searching questions about whether surveillance was impeding on people's civil liberties.
The latest twist came on Sunday morning when Greenwald's Brazilian partner David Miranda was arrested by British authorities at Heathrow Airport.
Scotland Yard has defended its use of anti-terrorism laws to detain the Brazilian national, despite the decision being challenged by David Anderson QC, the independent reviewer of terrorism legislation.
Guardian editor Alan Rusbridger said Miranda's detention – allied with the seizure of the hard drives – showed security forces were posing a very real threat to investigative journalism.
In today's edition he said the intervention from the Cheltenham intelligence agency had left him bemused.
He said: "One of the more bizarre moments in the Guardian's long history occurred - with two GCHQ security experts overseeing the destruction of hard drives in the Guardian's basement just to make sure there was nothing in the mangled bits of metal which could possibly be of any interest to passing Chinese agents.
"'We can call off the black helicopters,' joked one as we swept up the remains of a Macbook Pro."#
He added that the destruction of the hard drives missed the point that the investigation into the Snowden leaks could continue in the USA, away from UK law.
"It felt like a peculiarly pointless piece of symbolism that understood nothing about the digital age," he said.
"We will continue to do patient, painstaking reporting on the Snowden documents, we just won't do it from London."
Downing Street has come under increasing pressure to give an explanation for the decision to detain Miranda, whose laptop, phone and camera were seized, after the White House confirmed the decision to detain him was taken by British authorities.