Britains disgraced celebrity publicist
Clifford, 74, was more than three years into an eight-year jail sentence for multiple sexual assaults against teenagers. He died in hospital, a spokeswoman for the Prison Service said.
Clifford had collapsed twice in recent days at Littlehey Prison in Cambridgeshire, northeast of London, according to reports.
"As with all deaths in custody, there will be an investigation by the independent Prisons and Probation Ombudsman," the prison spokeswoman said.
"Our condolences are with Mr Clifford's family at this difficult time."
Clifford's daughter, Louise, 46, told Britain's Mail on Sunday he had collapsed in his cell on Thursday, and again on Friday.
After seeing a nurse, he was transferred to a local hospital where he suffered a cardiac arrest on Friday, and had been placed in a critical care unit, the newspaper reported.
"It was just too much," his daughter added.
Clifford, convicted in 2014, was the first high-profile figure jailed under Operation Yewtree, a police investigation of past sex offences following the scandal involving the late British TV presenter Jimmy Savile.
Clifford's victims came forward following revelations in 2012 that Savile -- a household name in Britain in the 1970s and 1980s -- had been a serial sex offender across the decades.
The public relations guru was eventually found guilty of eight indecent assaults on four impressionable young women aged 15 to 19 between 1977 and 1984.
Clifford high-profile career saw him forge a combination of media promotion and protection for his star clients, while becoming the go-to man for anyone with a celebrity scandal to sell to the press.
He helped promote The Beatles for record label EMI before starting his own firm aged 27.
He also looked after big US stars in Britain, including singer Frank Sinatra, boxer Muhammad Ali and actor Marlon Brando.
The stories he brokered included allegations about the sex lives of football icon David Beckham; former deputy prime minister John Prescott; and actor Jude Law.