Sullivan, 77, who has been the club's majority shareholder since purchasing a 50 per cent stake alongside the late David Gold in January 2010, issued a statement through his lawyers announcing his departure and pledging to pursue legal action against the BBC and any other outlet that repeats what he characterises as libellous claims.
"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."
He was pointed about his view of the process that had led to publication.
"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."
Sullivan was clear that the decision to resign was made out of care for the club rather than any admission of wrongdoing.
"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. 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."
West Ham United confirmed the departure in a brief club statement: "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."
Sullivan's departure comes at one of the most difficult periods in the club's recent history. West Ham were relegated from the Premier League last month after a dismal season at the London Stadium, and vice-chair Karren Brady — who had been at the club since 2010 alongside Sullivan — departed in April. The club won the UEFA Conference League in 2023, their first major trophy in 43 years. Sullivan said he would not make further public comment but reserved all legal rights in relation to any false or defamatory allegations.