David Cameron refused to comment today on reports that GCHQ - Britain's electronic eavesdropping agency - mounted an intensive spying operation on foreign politicians attending two G20 summit meetings in London in 2009.
The reports have the potential to embarrass the Prime Minister as he hosts leaders from the world's most powerful countries at the G8 summit in Northern Ireland - the biggest gathering of international statesmen to take place in the UK since the London G20.
He may face awkward questions from guests including US President Barack Obama, Russia's Vladimir Putin and the leaders of Germany, France, Italy, Canada and Japan over whether their communications will be secure during the two-day G8 meeting at Lough Erne, which begins today.
The Guardian said leaked documents show that in 2009 delegates had their computers monitored and phones intercepted on the orders of the British Government.
The intelligence agencies were even said to have set up internet cafes specifically to enable them to read the emails of those taking part in the summit.
Asked whether he could guarantee his guests that no similar operation was in place at Lough Erne, Mr Cameron told Sky News: "We never comment on security or intelligence issues and I am not about to start now. I don't make comments on security or intelligence issues - that would be breaking something that no government has previously done."
According to the Guardian, traditionally friendly powers such as South Africa and Turkey were among the countries targeted during the course of the operation in 2009.
The paper said the documents suggested that the operation was sanctioned at a senior level in the government of then prime minister Gordon Brown and that the intelligence obtained was passed to ministers.
Details of the eavesdropping were contained in documents obtained by Edward Snowden, the US National Security Agency (NSA) whistle-blower responsible for a string of disclosures about US intelligence operations.
During the course of the two summits in April and September 2009, GCHQ deployed what one document said were "ground-breaking intelligence capabilities" to monitor the communications of the visiting delegation.
They were said to have enabled a team of 45 analysts to be provided with live round-the-clock summaries of who was phoning whom during the proceeding.
The methods used included penetrating security of delegates' BlackBerrys in order to monitor their emails and phone calls.
Internet cafes were set up by GCHQ and MI6 able to "extract key logging info" for delegates, and provide the agencies with "sustained intelligence options against them" even after the summits were over.
The Turkish finance minister - and possibly 15 other members of his party - were among those targeted, according to the documents.
There was what the Guardian described as a sustained campaign to penetrate the computers of the South Africans - which had access to their foreign ministry network.
British intelligence also received reports relating to an NSA attempt to listen in on the phone calls of Russian leader Dmitri Medvedev as they passed through the satellite links to Moscow.
A briefing paper prepared for GCHQ director Sir Iain Lobban, dated January 9 2009, in preparation for a meeting with then foreign secretary David Miliband, set out the government's priorities for the G20 leaders' summit in April of that year.
"The GCHQ intent is to ensure that intelligence relevant to HMG's desired outcomes for its presidency of the G20 reaches customers at the right time and in a form which allows them to make full use of it," it noted.
A week after the September summit of finance ministers, an internal review concluded: "The call records activity pilot was very successful and was well received as a current indicator of delegate activity...
"It proved useful to note which nation delegation was active during the moments before, during and after the summit. All in all, a very successful weekend with the delegation telephony plot."
There was no immediate response from the Government to the report.
GCHQ previously said in relation to an earlier report about its involvement in NSA operations that its work was carried out "in accordance with a strict legal and policy framework which ensures that our activities are authorised, necessary and proportionate".