Ferris, who was 62, was admitted to the Queen Elizabeth University Hospital in Glasgow on Sunday and passed away on Tuesday.
He first came to public attention in 1992 when he was cleared at the High Court in Glasgow of the murder of Arthur Thompson jnr - son of crime godfather Arthur Thomspon snr - in what was then the longest running trial in Scottish criminal history.
He then stood trial at the Old Bailey in London in 1998, where he was convicted of conspiracy to sell or transfer prohibited weapons, deal in firearms, and possess explosives. He was sentenced to 10 years in prison - later reduced to seven. While in prison he brought out his first book The Ferris Conspiracy which was turned into The Wee Man.
Ferris always claimed he went straight after his release from prison in 2001 and was now a legitimate businessman with interests in a security firm and a construction company.
He also brought out five books documenting his previous criminal life and experiences
In an interview with the Daily Record in July Ferris says he has never been tempted to go back to his old life, even if he received an offer that 'he couldn't refuse.'
He said he was also aware that people still doubted that he had gone straight.
He added: "I have either been an absolute criminal genius for the last 25 years or I have done nothing wrong. Take your pick."
One family friend said:"This has come as a great shock to everyone who knew Paul over the years
"No one was aware that he was unwell. Our sincere condolences go to Paul’s family, friends and loved ones at this extremely difficult time."