A man posing as a police officer became abusive and grabbed his crotch after a woman said she did not believe him.
The man approached a woman at Hampstead Heath overground station in South End Green at around 9.30am on Monday, March 11.
The man claimed to be a police officer, British Transport Police (BTP) said.
When the woman said she did not believe him he became agitated and abusive and threatened her while grabbing at his crotch.
Officers have released a CCTV image of a man who they believe could help with their investigation.
If anyone recognises him, or has any other information, they should contact BTP by texting 61016 or call 0800 40 50 40 quoting reference 236 of 11 March.
Alternatively contact Crimestoppers anonymously on 0800 555 111.
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