Michael Mansfield QC said that Diana had a "credible and understandable basis" for her fear that she was being spied on because of the threat she posed to the Royal Family and the Establishment.
"I am on record as saying that I believe the deaths were the result of murder," the Daily Express quoted Mansfield, 68, as saying.
"There are a combination of motives for the desire to see Princess Diana and Dodi killed, many of which have been regularly rehearsed by Mohamed Al Fayed," he said.
Public concern over the possibility that a conspiracy lies behind the deaths of Diana and Dodi Fayed on August 31, 1997, has propelled Mansfield's autobiography, 'Memoirs Of A Radical Lawyer', into the bestseller charts.
In his book, Mansfield describes his role at the inquest into the deaths of Diana and Dodi in a Paris underpass.
"I found it difficult simply to accept that what happened in Paris was 'just one of those tragic things'," he wrote.
"Judging whether a hidden hand is at work is always difficult, but I prefer a healthy and inquisitive assessment of the authorised version.
"I have always believed that whatever had caused the crash, it was not an accident. And, as it transpired, that belief was shared by the jury at the inquest," he stated.
His book also highlights other issues never "resolved by evidence, or reflected in the verdict".
They include the fate of a box of personal papers belonging to Diana, the mystery driver of a white Fiat said to be involved, what chauffeur Henri Paul was doing before the crash and how he came to have large sums of money paid into his bank accounts before the crash.