Hasnat Khan, a 54-year-old heart and lung surgeon, learned last year from Scotland Yard that his voicemail may have been hacked in 2007, shortly before the inquest into the princess' death, The Daily Telegraph reported Saturday.
A friend of Khan's, whose name was not reported, says Khan believes his phone may have been hacked as early as 1996, while he was engaged in an affair with Diana.
"It is only when Dr. Khan started to think back to the time when he was with Diana that it began to occur to him that someone may have been trying to listen in to his messages," the friend said. "Their friendship was very private and very discreet but he now fears journalists were targeting him in a bid to find out what was going on."
Khan is seeking substantial damages from News International, The Daily Telegraph said.
He has said he plans to donate any money he receives to a heart unit he set up in a Pakistani hospital.
Khan and Diana in met 1995 at a hospital in England where a friend of hers was recovering from heart surgery. The two began an affair that ended shortly before the princess' August 1997 death in Paris, The Daily Telegraph said.