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Concern grows over foreign involvement in spy's death

The 31-year-old maths and computer expert would have been regarded as a valuable asset for his knowledge of the inner-workings of GCHQ, the government's listening post in Cheltenham, and for his work on preventing cyber attacks on the banking and transport infrastructure.

It is understood Mr Williams's job at the time of his death was creating computer defences in the City of London. Williams would have had access to information which other countries would want to obtain.

The intelligence source said: "His job was to defend the banking system on which Britain's banking, commerce and all our public services depend. It was the kind of work that would have made him prime target for recruitment.

"He was also in a position to know about huge money transfers out of the Middle East which were linked to terror groups. It would be priceless data."

One theory being examined is that Mr Williams may have had an approach from a rival agency, and either rebuffed it without informing his superiors or initially agreed to co-operate then got cold feet.