David Sullivan has stepped down from his position as joint-chair of West Ham.
The 77-year-old, who had been on the board at West Ham for over 16 years, has stepped down ahead of the "impending publication of serious historic allegations". Through West Ham and his own statement, Sullivan denied all allegations.
It is understood none of the allegations relate to West Ham or any of its operations, the club said.
Sullivan's statement read: "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.
"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.
"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. I am absolutely not the person the media has decided to paint me as.
"I have not been provided with any proper explanation as to how these individuals or their claims were independently verified or assessed for credibility prior to publication.
"I believe that the entire process has been fundamentally unfair and completely lacking in any due impartiality. I will be suing the BBC for libel, along with any other media outlet that repeats any libellous allegations.
"None of these allegations relate to my more than 30 years in football; West Ham United has been one of the greatest passions and privileges of my life. I care deeply about the club, its supporters, its players, its staff and its future.
"At what is already a challenging and important time for the club, I refuse to allow personal matters concerning me to become an unnecessary distraction or a source of instability.
"Therefore, after very careful consideration and with a heavy heart, I have decided to resign as joint-chair and director of West Ham United FC with immediate effect.
"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.
"I am stepping down to apply my full energy and attention on fighting these false allegations. I have officially engaged my legal team and we are preparing to issue legal proceedings in respect of each and every false complaint that has been made against me. I will be seeking full redress, legally and publicly, on every single front.
"As I depart, I want to thank everyone connected with West Ham United, past and present, for the loyalty, friendship, memories and European adventures and successes that I will carry with me forever.
"West Ham United will always remain close to my heart, and I have total confidence that the board of directors and interim chief executive officer Karim Virani will do everything they can to take the club forward in the right direction. I wish everyone connected with the club every success in the future.
"I will not be making further public comment at this stage, but I reserve all legal rights available to me in relation to any false or defamatory allegations."
A BBC spokesperson said: "BBC Panorama and The Times newspaper have been working together on a joint investigation into the behaviour of David Sullivan, who has announced he is stepping down as joint-chair and director of West Ham United FC.
"Over decades, he's made millions from pornography, newspapers and football. The investigation is due to be broadcast and published on Monday."
A statement from West Ham released on Saturday read: "West Ham United can confirm that joint-chair David Sullivan has stepped down from his position with immediate effect.
"Mr Sullivan has also resigned as a director of both WH Holding Limited and West Ham United Football Club, having been made aware of the impending publication of serious historic allegations.
"It is understood none of the allegations relate to West Ham United or any of its operations.
"Through his own legal representatives Mr Sullivan denies any illegal conduct and has taken the decision to step down in order to avoid disruption to the club while he addresses the matter privately."
The statement also confirmed that Virani will continue to be responsible for leading the club's day-to-day operations and will report into the remaining board of directors.
West Ham were relegated from the Premier League last month.
