GCHQ, Britain's electronic eavesdropping agency, is said to have spied on foreign politicians
A diplomatic row has broken out over claims that Britain spied on foreign governments attending G20 meetings in London in 2009.
Turkey's foreign ministry demanded answers over reports its delegates had computers monitored and phones tracked.
South Africa condemned the alleged "abuse of privacy" and a senior Russian politician said it was a "scandal".
PM David Cameron would not comment on the claims, said to be based on leaks by a former IT contractor.
It comes as the G8 summit of eight of the world's leaders gets under way in Northern Ireland.
All of the nations attending the G8 summit also attended the 2009 meetings which were said to have been the target of UK intelligence agencies.
'Trust and transparency'
The claims, reported by the Guardian newspaper, are that GCHQ - Britain's electronic eavesdropping agency - spied on foreign politicians and officials attending two G20 summit meetings in April and September 2009.
UK intelligence agencies set up internet cafes specifically to enable them to read the emails of those taking part in the summit, the paper quoted the leaked documents supplied by Edward Snowden - a former contractor to the US National Security Agency - as saying.
The British authorities are expected to present an official and satisfactory explanation on this issue"
Turkey Ministry of Foreign Affairs
It alleged that the operation was sanctioned at a senior level in the government of then prime minister Gordon Brown and the intelligence obtained was passed to ministers.
The Turkish, South African and Russian delegates are said to have been among those targeted.
In a statement, Turkey's Ministry of Foreign Affairs said: "The claims published in today's edition of the Guardian that phone conversations of our Minister of Finance Mehmet Simsek and his delegation were tapped during their visit to the United Kingdom in 2009, on the occasion of the G20 meeting, are alarming.
"If there is even the slightest truth to any of these claims contained in this news report, this will evidently constitute a scandal, primarily for the country concerned.
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Gordon CoreraSecurity correspondent, BBC News
There can be a fallout diplomatically when any type of spy operations is exposed - you only have to ask the US CIA officer who had his face splashed on TV in Moscow recently to see that.
When Soviet Premier Nikita Khrushchev came to Britain on a state of the art warship in 1956, Prime Minister Anthony Eden said he wanted no intelligence operations carried out against him for fear of diplomatic embarrassment.
And yet British intelligence still ended up allegedly bugging his hotel room in Claridges and sending a frogman into Portsmouth harbour to inspect at the ship. The frogman died in the operation, causing a major intelligence scandal.
This time everything may well have been properly authorised but that will only focus the embarrassment more on the politicians who signed off. This, after all, is not the Cold War.
Countries will certainly protest but many of them will also be doing the same to foreign officials visiting their own shores. And those here for the G8 summit might just be thinking twice about what they say on the phone or write in an email.
"In an environment where mutual trust, respect and transparency should be essential for international co-operation, such act by an allied country would clearly be deemed unacceptable, should the news report turn out to be true.
"The British authorities are expected to present an official and satisfactory explanation on this issue. As a matter of fact, necessary diplomatic initiatives have been taken in this regard."
The South African government said it had noted reports of alleged attempts campaign to access its computers "with concern".
"We do not yet have the full benefit of details reported on but in principle we would condemn the abuse of privacy and basic human rights particularly if it emanates from those who claim to be democrats," it said.
"We have solid, strong and cordial relations with the United Kingdom and would call on their government to investigate this matter fully."
The Guardian also claimed that GCHQ received reports from a US attempt to listen in to a call being made via satellite to Moscow by the then Russian president, Dmitry Medvedev.
The allegation prompted Alexei Pushkov, the chief of the foreign affairs committee in the lower house of the Russian parliament, to write on Twitter: "It's a scandal... The US denies it, but we can't trust them".
The UK Foreign Office (FCO) confirmed Turkey had raised the claims with Britain's ambassador to the country, Sir David Reddaway.
A spokeswoman said: "In line with longstanding practice we do not comment on intelligence matters.
"We can confirm that the Turkish Ministry of Foreign Affairs has raised this issue with the ambassador."
The documents allegedly identified Turkish Finance Minister Mehmet Simsek as a target
When asked if the ambassador had been summoned to the foreign ministry, as some reports suggest, the spokeswoman said: "No, this was discussed in a phone call."
Details of the eavesdropping were allegedly contained in documents obtained by Mr Snowden, who was behind a series of disclosures about the National Security Agency in Washington.
Asked whether he could guarantee that no similar operation was taking place at the G8 summit, Mr Cameron said he never commented on security and intelligence issues.
BBC security correspondent Gordon Corera says there is no doubt states spy on each other - Britain will be spying on North Korea and Iran to gather information about their nuclear programmes, for example.
"But when you get into countries that are a bit more friendly, [for example] Nato allies like Turkey, that is where the sensitivities really hit home and that is what I think is particularly significant about this leak of information," adds our correspondent.