The billionaire co-owner of Chelsea, Hansjorg Wyss, has been accused of sexually harassing an employee at his California winery.

The 89-year-old allegedly made unwanted advances to Madison Busby, a 30-year-old who took on a job at Wyss’s Halter Ranch winery, which was managed by her now-husband Bryce Mullins.

Busby alleges that Wyss visited the winery in September 2019 and suggested that the three cook together. “While Ms Busby cooked at the stove, Mr Wyss, then in his mid-eighties and married, deliberately placed his hand on Ms Busby’s butt and groped her,” the lawsuit says.

Wyss allegedly also asked Mullins if he had “sexy” pictures of Busby on his phone. Busby, an architectural engineer graduate, says that she was “appalled and disgusted” but did not complain because she feared that Mullins would lose his job.

Halter Ranch said that the allegations were untrue. “We intend to vigorously advance the facts that surround their time at the winery and their departure,” a statement said.

According to the complaint filed in San Luis Obispo County, Busby moved into the winery with Mullins in 2020 and they were visited by Wyss and his late wife.

“During their stay, Mr Wyss made several sexual propositions of Ms Busby,” the complaint says. “In addition, Mr Wyss told stories of his sexual exploits in graphic detail to Ms Busby and Mr Mullins. These included tales of his past and ongoing sexual affairs with women.”

Busby said she became an employee at Halter Ranch in January 2021. “After Ms Busby was hired, Mr Wyss began to proposition Ms Busby and Mr Mullins to have sex with him, either as a group or with Ms Busby alone, and Mr Wyss began trying to insert himself into their sex lives,” says the lawsuit.

“Ms Busby attempted to ignore the comments, change the subject or deflect them. At the time, she did not raise complaints with Mr Wyss out of concern for their employment.” Busby left the winery in July 2024 and is now seeking compensation.

In a statement denying the allegations, Halter Ranch added: “For almost five years, starting in 2019, Mr Mullins and his current wife voluntarily made themselves part of the Halter Winery community and took advantage of its owner’s generosity.

“This included deciding to become employees of the winery, choosing to live at the winery rent free for years, frequently travelling with the owner to Europe, the Caribbean and elsewhere at the owner’s expense, asking the owner and his wife to host their wedding party and inviting the owner to serve as best man.

“Through all these years, they never complained about the owner’s conduct, or simply declined to spend so much time with him, until after they voluntarily left their employment at the winery in 2024.”

Wyss built his fortune in the medical device industry and, according to Forbes, is worth $4.8 billion. He established the Wyss Foundation in 1998, a charity which aims to “confront the global conservation crisis”.

In 2022, Wyss was part of the consortium that bought Chelsea after sanctions were imposed on the Russian billionaire Roman Abramovich.