National Security Agency leaker Edward Snowden says the British electronic surveillance agency GCHQ reportedly hacked into the phones and emails of foreign politicians and officials.
The London Guardian reported the agency did that during two major international summits in London four years ago.
The paper said the disclosure raises new questions about the surveillance programs from the United Kingdom and the United States.
The allegations became public only hours before Britain was due to open this year's G-8 summit. Some visiting delegates may want to know if the UK spied on them in 2009.
Prime Minister David Cameron told G-8 leaders he would not be discussing the matter.
"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."
Meanwhile, it's unclear how Snowden gained access to British intelligence documents, but the Guardian mentioned that the material came from a top secret network shared by GCHQ and the NSA.