A key UK security industry source who served in the military told the Sunday Express that GCHQ remotely switched on recorder modes right up to the moment the couple took their fatal Paris car trip.
If made public, the phone recordings could help throw light on sensational claims by former SAS serviceman Soldier N that Diana was murdered by an SAS assassination squad.
The bugging claim came as a separate Sunday Express investigation revealed that film footage of Diana’s last hours was “kept secret”. An exhaustive inquiry in the French capital has confirmed the existence of CCTV records of the night Diana died in August 1997.
Many belong to private companies who used France’s strict privacy laws to avoid having to hand them over to the police.
Just as crucially, a key traffic camera overlooking the scene of the crash in the Alma Tunnel was said to be switched off or malfunctioning.
However, one operator contacted 16 years on said: “Images would have been available if people wanted them to be. The truth is that every excuse possible was made to make sure that live film could be kept secret.
“This suited lots of powerful people, especially those who wanted to dismiss the crash as a simple traffic accident.”
The operator, who asked not to be identified because he “fears for my safety”, said he was convinced that all available film was “rounded up and hidden or destroyed”.
Military sources have claimed Diana’s driver, Henri Paul, was blinded with an intense flash of light forcing him to lose control of their Mercedes on an underpass after leaving the Ritz hotel.