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 were said to have 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 as saying.
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.
The reports are based on documents obtained by American whistleblower Edward Snowden.
Turkey has described as “scandal” reports that the UK government spied on foreign governments participating in G20 meetings in London in 2009.
The Turkish Foreign Ministry called for explanations from Britain over reports Turkish delegates were among those who had their computers monitored and phones intercepted.
"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”, it said in a statement.
"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”, the statement said.
"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”, it added.
"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 UK Foreign Office (FCO) said Turkey had raised the claims with Britain's ambassador to the country, Sir David Reddaway.
The claims, reported by the