Starbucks says it is investigating claims that an employee was mistreated by co-workers because he is gay.

A customer at the coffee giant’s Centereach, Long Island, store claims she saw three female staff berating the man for 15-20 minutes in a “brazen” display of homophobia.

Missy Allen, who is a lesbian, says she was in the store with her three-year-old daughter Lily when the incident happened.

She wrote on her wife’s blog: “I have never, in my entire life seen such a gross and unapologetic display of ignorance and intolerance. I was horrified that my daughter was exposed to that.”

Ms Allen claimed that she heard the female store manager call the employee ‘Jeffrey’.

She wrote: “[The manager] told him that they were not interested in his politics or beliefs and his thoughts were down right offensive to his co-workers. They did not want to hear about his personal life.

“When Jeffrey pointed out that they ALL talked about their personal lives … [he was told] that his beliefs were not welcome at Starbucks. She went on, and on and on talking about leadership building workshops where she learned to ‘Keep it to herself’.

“She kept reminding him, ‘You are not fired but …” As if to say, you are not fired but you are really not welcome here anymore.”

Ms Allen said she did not know the context of the conversation, but added: “[I] know that worker was attacked and humiliated on the middle of your shop floor. I don’t care what his offence was, that sort of business should be conducted in a back room.

“She was bigoted, intolerant, insensitive and no matter how upset she was at Jeffrey, her comments and sentiments should not have been overheard by one of your customers, gay or not.”

Starbucks says it is investigating. Spokesman Allan Hilowitz said that Ms Allen had got some parts of her account wrong but would not give specific examples.

He said the conversation was a performance evaluation and the employee chose to leave.

The coffee chain has not said whether anyone will face disciplinary action and a further statement said: “At Starbucks, we pride ourselves on being a great place to work. We are deeply dedicated to our core values – to embrace diversity and treat each other with respect and dignity.”