Dressed in the fast food icon’s familiar white suit and black bow tie, the actor evaded tight security to gain access to the restricted areas of the complex.
He even posed for a photograph with Ali Treki, the new president of the UN General Assembly, before the alarm was raised and he was ejected.
A spokeswoman for Ban Ki-moon, the UN secretary-general, yesterday said that an investigation had been launched into the security breach, which was dreamed up by KFC as a promotional stunt.
“It should not have happened – that I will stress, and very strongly,” Michele Montas told Canwest News Service, the Canadian news agency.
“There was some lapse in security and the individual in question was, on the initiative of one security guard, taken into the UN.”
As part of its campaign to promote a new menu range, KFC is “lobbying” the UN for the fictional Grilled Nation to be accepted as a member state.
The fast food giant has written to Mr Ki-moon personally asking for grilled chicken lovers to be be represented at the assembly, in a letter dismissed by Mrs Montas as “absolutely void”.
A spokesman for Dr Treki, a former Libyan foreign minister, denied that he held a meeting with the Colonel Sanders impersonator, claiming that he only shook his hand out of courtesy during the incident on Thursday.
The actor, named by KFC as Robert Thompson, also posed for photos in the main General Assembly hall beneath the UN logo.
The real Colonel Sanders died in 1980, half a century after devising the “secret recipe” of herbs and spices that is still used for cooking chicken in KFC restaurants today.