Britain has been asked by Belgium to respond to claims GCHQ hacked in the IT network of Belgian telecoms provider Belgacom.
Federal prosecutors said in September they were investigating suspected foreign state espionage against Belgacom, the dominant telecoms provider in Belgium and also a top carrier of voice traffic in Africa and the Middle East.
German magazine Der Spiegel reported later that the British electronic surveillance agency GCHQ had placed a virus in Belgacom's network.
"Following the article in Der Spiegel the government asked Belgian intelligence services to ask their British colleagues for more information," a source close to the government said.
A GCHQ spokeswoman said: "We are not making any comment."
Belgacom said at a hearing before the European Parliament yesterday evening that it had removed the virus from its systems and that an investigation into who was responsible was ongoing.
A representative of GCHQ was scheduled to speak at the same hearing but did not appear.
At the time the incident came to light, federal prosecutors said in a statement that the former state telecoms monopoly Belgacom had filed a complaint in July about the hacking of several servers and computers.
"The inquiry has shown that the hacking was only possible by an intruder with significant financial and logistic means," they said.
"This fact, combined with the technical complexity of the hacking and the scale on which it occurred, points towards international state-sponsored cyber espionage."
Prosecutors said the intruder had used malicious software and advanced encryption techniques with the aim of gathering strategic information, and had not sabotaged Belgacom's data or sought to cause direct economic damage.