A man entered the Beer House at Waterloo railway station and ordered a drink at the bar where the barmaid noticed he was unsteady on his feet.
She instructed the other staff not to serve him any more alcohol and told him he would not be served another drink.
The man began to verbally abuse her with racial comments before leaving the pub.
The police have launched an appeal to help find the man in connection with the incident which happened just after 7pm and have released a CCTV image of him.
Investigating officer PC Rachel Parfitt said: "I am keen to speak to the man in the CCTV image because I believe he holds vital information to the investigation."
She added: "The barmaid was simply going about her job.
"Nobody should be made to feel intimidated and everyone has the right to work in a safe and secure environment."
Anyone with information can call British Transport Police on 0800 405040 or text 61016 quoting reference 216 of 23/05/16.