The PC, named as 31-year-old Philip Juhasz, allegedly told the Pakistani worker to "you are wasting our food in this country" and exposed him to a barrage of expletives while off-duty and drunk.
He now faces the sack for gross misconduct after being convicted of a racially aggravated public order offence.
The case is the latest in a string of revelations relating to racism within Scotland Yard, as it was revealed just one officer out of 120 found guilty of racist behaviour had been dismissed in the last ten years.
Figures, disclosed under the Freedom of Information Act, showed just six of the officers were forced to resign with 21 others receiving some form of sanction such as a fine.
The statistics, which refer to complaints made between 1999 and 2011, came to light shortly after Scotland Yard confirmed it had suspended eight officers amid allegations of racism.
Ten new cases have been referred to the Independent Police Complaints Commission, bringing the total to 20 officers and one police worker.
The report, by then-commander Brian Paddick, said African-Caribbean people were being stopped too often by officers and added it could result in alienating and causing discontent in minority communities.
Met Commissioner Bernard Hogan-Howe has now stressed his determination to stamp out racism in the force, and urged officers to report any colleagues who made inappropriate remarks.
The victim of the latest case, 23-year-old Umer Nazir, has now told how he suffered abuse while working late at the Delice de France snack kiosk at King's Cross station last September.
He said the drunken officer demanded to be sold half price food that was about to be thrown away.
When he refused, he claimed, the officer pulled out his warrant card and said: 'Do you know who I am? I'm a police officer'.
After asking where he was from, Mr Nazir reportedly said: 'He said, "Oh, I'll tell you where you're from - you're from Pakistan, and your ******g family are dying from lack of food over there.
"You are wasting our food in this country, you ******g w*****r. Go back to your ******g country"."
Mr Nazir claimed two uniformed officers who were passing by let the suspect go after he showed them his warrant card, while others gave good character witnesses in court.
Officers from British Transport Police responded to a 999 call and retrieved CCTV footage of the officer gesticulating.
Juhasz was arrested and appeared in court, where he was convicted and fined £1,000.
He is now suspended from duties and will face further disciplinary proceedings within the next few weeks where he could be dismissed.
A spokesman for the Met said: "An officer was convicted at court on 23 March 2012 of racially aggravated public order offences that were investigated by British Transport Police.
"This is a mandatory referral to the IPCC. The officer is suspended."
Senior black officers in the Metropolitan police have said warnings about growing prejudice have been ignored, even 13 years after the Stephen Lawrence case led to accusations of institutional racism.
Leroy Logan, who founded the Met Black Police Association, said: "Like so many things, it lands on deaf ears until such a time as a free press - the media - get hold of it and forces people into action.'