Hacking of computers at Belgian telecoms firm Belgacom, alleged to have been carried out by a British spy agency, was more far-reaching than previously thought and went undetected for more than two years.

News of the intrusion into Belgacom's networks first broke late last year when Belgium asked Britain, its NATO and European Union partner, to respond to allegations that its intelligence service was responsible. It said the malicious software found on Belgacom's systems was one of the most advanced spy tools ever identified by security researchers. Belgacom, Belgium's dominant telecoms provider, was seen as a top target by the British spy agency.