When he left the Microsoft board last year, Gates said he was stepping down to focus on philanthropy.

Formerly the world’s richest person, Gates’s fortune is estimated at well over $100bn.

The news comes not long after he and his wife, Melinda Gates, announced a divorce after 27 years but said they would keep working together at the Bill and Melinda Gates Foundation, one of the largest charitable foundations in the world.

Citing anonymous sources, the Journal said board members hired a law firm in late 2019 to conduct an investigation after a Microsoft engineer alleged she had a sexual relationship with Gates over several years.

Citing another person familiar with the matter, the Journal said Gates resigned from the board before the investigation was finished.

An unnamed spokeswoman for Gates acknowledged an affair almost 20 years ago, which ended “amicably”.

The spokesperson said Gates’s “decision to transition off the board was in no way related to this matter”.

In an email to the Associated Press, Microsoft said it “received a concern in the latter half of 2019 that Bill Gates sought to initiate an intimate relationship with a company employee in the year 2000.

“A committee of the board reviewed the concern, aided by an outside law firm, to conduct a thorough investigation. Throughout the investigation, Microsoft provided extensive support to the employee who raised the concern.”

Also on Sunday, the New York Times reported that Gates had developed “a reputation for questionable conduct in work-related settings”.

Citing sources it said had direct knowledge of such behaviour, the Times reported that on at least a few occasions, Gates made overtures to women who worked for him at Microsoft and the Bill and Melinda Gates Foundation.