According to a report in the Guardian, British agency GCHQ repeatedly hacked into diplomats' phones and emails when the U.K. hosted the 2009 G-20 summit in London.
The Guardian cites several internal government documents provided by former National Surveillance Agency contractor Edward Snowden as the basis for these new allegations. The documents also reportedly show that the NSA tried to eavesdrop on Russian leader Dmitry Medvedev as he telephoned back to Moscow via satellite. The NSA is the GCHQ sister organization in the U.S.
Snowden is hiding in Hong Kong after leaking to The Guardian that the U.S. government had collected phone records of millions of Verizon customers.
It's not clear how Snowden would have had access to the British intelligence documents, although the Guardian says that source material was drawn from a top-secret internal network shared by GCHQ and the NSA. British academic Richard J. Aldrich said he wouldn't be surprised if the GCHQ material came from a shared network accessed by Snowden. According to Aldrich, the NSA and GCHQ collaborated so closely that in some areas the two agencies effectively operated as one.
Meantime, Snowden fielded questions Monday in a live online forum hosted by the Guardian. Snowden was asked if information would continue to leak "if anything happens" to him.
"All I can say right now is the U.S. government is not going to be able to cover this up by jailing or murdering me," Snowden reportedly wrote back. "Truth is coming, and it cannot be stopped."
Prime Minister David Cameron would not comment on the allegations as the G-8 summit began.
"We never comment on security or intelligence issues and I am not about to start now," he said. "I don't make comments on security or intelligence issues. That would be breaking something that no government has previously done."
GCHQ also declined to comment.
The news prompted an angry response from Russian officials concerned that their communications had been intercepted. Russian officials complained Monday about reports that their delegation had been snooped on in earlier summits.
"It's a scandal! The U.S. and British special services tapped (then President Dmitry) Medvedev's phone at the 2009 G-20 summit. The U.S. denies it, but we can't trust them," Alexei Pushkov, the Kremlin-connected chief of foreign affairs committee in the lower house of Russian parliament, wrote on his Twitter feed Monday.
Sergei Devyatov, a spokesman for the Federal Protection Service that provides security for top Russian government officials, said in a statement carried by the Interfax news agency: "The Federal Protective Service is taking every necessary measure to provide the appropriate level of confidentiality of information for top-ranking officials of the country."
ABC News and The Associated Press contributed to this report.