Britain intercepted phone calls and monitored computers used by officials taking part in two high-level international finance meetings in London in 2009, a British newspaper has claimed.
The Guardian said some delegates from countries in the Group of 20 used internet cafes that had been set up by British intelligence agencies to read their emails.
The G20 comprises top economies from around the world.
The report was published hours before leaders of the Group of Eight (G8) countries start a two-day summit in Northern Ireland.
A spokesman for Britain's foreign ministry declined to comment.
The Labour party, which held power in 2009, was not immediately available for comment.
The Guardian said it had seen classified documents that detailed secret monitoring by British intelligence of officials at a G20 leaders summit and a finance ministers' meeting in 2009.
It suggested it had been sanctioned at a senior level by the government of former British prime minister Gordon Brown.
The aim of the monitoring, which included tracing who was calling who, appeared to have been to get an edge in the meetings and targets included South Africa and Turkey, the report said.
The Guardian this month reported details of surveillance by the US National Security Agency (NSA) of phone records and internet data in the United States.
The newspaper said the evidence was contained in documents that were leaked by the former NSA contractor Edward Snowden and seen by the Guardian.