The incident allegedly happened in 2008 when the woman was working in the same building as the corporation’s office in Los Angeles, BBC News reported.
The comedian is then said to have joked about it on air as his Radio 2 co-presenter Matt Morgan said Brand “showed his willy to a lady”.
The woman was reportedly left “stunned and disgusted” by the incident.
The 48-year-old comedian and actor has been accused of rape, assault and emotional abuse between 2006 and 2013, when he was at the height of his fame working for the BBC and Channel 4 and starring in films.
Brand denies the allegations, which also include claims of controlling, abusive and predatory behaviour.
The woman at the centre of the latest claims, given the pseudonym Olivia by the BBC, said she had come forward because of the claims that emerged on Saturday.
She said went to the bathroom to get some sinus medication and was startled to find the comedian behind her. She said he called her Betty, then allegedly said: “Well, I’m gonna f*** you.”
“And I said: ‘No, you’re not.’”
She told the BBC he then pulled out his penis. She said: “No, no, no.”
The bathroom door was closed, and she felt trapped, she said.
Brand only left when someone from his team banged on the door, she recalled.
The encounter left her stunned, she said, adding: “It shouldn’t have happened, but who would believe me?”
Neither Brand nor his co-presenter replied to requests for comment by the BBC and the PA news agency.
In a statement, a BBC spokesperson said: “We’re very sorry to hear of these allegations and we will look into them.
“We are conducting a review to look at allegations of this nature and if the woman who has shared her story is willing to speak to us, we would be very keen to hear from her and anyone else who may have information.”
In 2019, BBC management was informed about the incident by a BBC staff member whom she had spoken to – but the woman said she never received an apology, and no action was taken.
Channel 4 has also announced an investigation into the presenter’s time at the channel. It has also removed all content featuring Brand, while the BBC said it has reviewed content and “made a considered decision to remove some of it, having assessed that it now falls below public expectations”.
Both YouTube, which hosts Brand’s video channel, and podcasting platform Acast, where his Under The Skin podcast appears, have said he would not make money from advertisements on their sites and apps.
The remaining shows of Brand’s Bipolarisation tour have also been postponed.
Brand’s last video to his platforms denied any criminal allegations and maintained all his relationships had been “consensual”.
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