The 77-year-old quit this week in what was a sudden statement from the Hammers in a move that comes after more than 16 years on West Ham‘s board and amid fan protests directed at him.
West Ham's statement
The club's statement 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.
"Interim Chief Executive Officer Karim Virani, reporting into the current Board of Directors, will continue to be responsible for leading the Club’s day-to-day operations.
"The Club will provide an update on the future structure of the Board of Directors in due course, but will make no further comment at this time."
Sullivan's statement
Sullivan also released a lengthy statement of his own, denying any wrongdoing as he plans to take legal action against the BBC over reports he claims are false.
"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 said in a statement of his own.
"The false allegations levelled against me have been sensationalised by the media.
"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.
On stepping down
Sullivan then spoke about stepping away from his role at West Ham, a decision he says hurt him deeply as he hopes the club can move forward in the right direction.
"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."