David Sullivan has stepped down from his role as joint-chair of West Ham United after being "made aware of the impending publication of serious historic allegations", the club has said.
It is understood none of the allegations relate to West Ham or any of its operations, the club added.
A statement from Mr Sullivan said: "I have recently become aware that factually incorrect and entirely false, decades-old allegations concerning my personal life are due to be broadcast and published."
He added: "The false allegations levelled against me have been sensationalised by the media. After a lifetime spent building businesses in the adult industry in which I have met thousands of women, it is sadly inevitable that a small number of improper conduct claims are being made against me. I categorically deny these claims."
Mr Sullivan made his fortune from owning pornographic magazines and sex shops.
"I am a private man, and those who personally and professionally know the real David Sullivan, not the caricature invented by the tabloids, know exactly who I am and what I stand for," he said.
"I am absolutely not the person the media has decided to paint me as."
Mr Sullivan bought a 50% share in West Ham in 2010 with his business partner, David Gold, who died in 2023.
Until his resignation on Saturday, Mr Sullivan was on the board of directors alongside joint-chairs Czech billionaire Daniel Kretinsky and Vanessa Gold.
Mr Sullivan and Mr Gold previously owned a stake in Birmingham City between 1993 and 2009, where Baroness Karren Brady served as the managing director.
Baroness Brady then served as vice-chair of West Ham for 16 years until she stepped down on April.
In his statement, Mr Sullivan said his decision to resign came as West Ham faces a "challenging and important time" and he didn't want to provide any distractions.
"This has been an incredibly painful decision to make, but it is one made out of love, respect, and responsibility toward a football club and a fan base that deserve absolute unity and focus moving forward," he said.
West Ham were relegated from the Premier League last month after 14 years in the top flight.
The club's demotion to the Championship was confirmed on 24 May despite a 3-0 win over Leeds.
The 2025/26 season saw the club sack Graham Potter as coach in September, after the team lost four out of five games.
Off the pitch, the season has been mired by protest, with fans waving red cards on the 16th minute of evert game, to mark the 16 years under Mr Sullivan, and marches protesting the club's leadership.
Fans criticised West Ham's move from Upton Park to the London Stadium in 2016, with some supporters arguing the club lost its identity in the switch.
(c) Sky News 2026: David Sullivan steps down as West Ham joint-chair over 'serious historic allegations'
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