A document cited by the London-based Guardian newspaper appeared to boast of GCHQ's ability to tap into smartphones.
The Guardian quoted the document as saying that "capabilities against BlackBerry provided advance copies of G20 briefings to ministers."
It went on to say that "Diplomatic targets from all nations have an MO (a habit) of using smartphones,'' adding that spies "exploited this use at the G20 meetings. ...''
The classified documents allegedly came from Edward Snowden, a former U.S. National Security Agency contractor.
The newspaper also reported that the British spies monitored conversations in real time, hacked emails, stole passwords and went so far as to set up a bugged Internet cafe in an effort to get an edge in the high-stakes negotiations.
On Monday, the Waterloo, Ont.,-based company defended its smartphones but said it was not going to comment on the report about possible government surveillance by The Guardian.
"While we cannot comment on media reports regarding alleged government surveillance of telecommunications traffic, we remain confident in the superiority of BlackBerry's mobile security platform for customers using our integrated device and enterprise server technology," BlackBerry said in a statement.
"Our public statements and principles have long underscored that there is no "back door" pipeline to that platform. Our customers can rest assured that BlackBerry mobile security remains the best available solution to protect their mobile communications."
The revelations broke at the start of the G8 Summit, a meeting of the world's leading economies, in Northern Ireland on Monday.
Russia, Turkey and South Africa, among reputed targets of the GCHQ spies, all called for an investigation into the report.
— With files from the Associated Press.
Also on HuffPost