The dish uses extra-hot chillies which are deliberately burnt

Firefighters wearing protective breathing apparatus were called to D'Arblay Street, Soho, after reports of noxious smoke filling the air.

Police closed off three roads and evacuated homes following the alert.

Specialist crews broke down the door to the Thai Cottage restaurant at 1900 BST on Monday where they discovered the source - a 9lb pot of chillies.

Nam Prik Pao recipe Heat garlic and shallots in oil and remove to a bowl Place red chillies in the pan with some oil and fry until they go dark in colour. Then set aside Mix shrimp paste with the rest of the ingredients and pound in a mortar and pestle Return the mixture to the heat until it becomes a thick dark coloured paste

The restaurant had been preparing Nam Prik Pao, a red-hot Thai dip which uses extra-hot chillies which are deliberately burnt.

But the smell prompted several members of the public to call the emergency services.

Alpaslan Duven, a Turkish journalist based in the restaurant's building, said: "I was sitting in the office when me and my chief start coughing and I said this was something really dodgy.

"I looked out of the window and saw people rushing and then we heard the sirens."

Supranee Yodmuang, the restaurant supervisor, was above the restaurant when she received a phone call from her boss.

"It was about 4pm when I saw the police who were closing off the roads but I didn't know why.

"My boss rang me and said I had to get out of the building because of a chemical attack."

She added: "Because we're Thai, we're used to the smell of chillies."

A Scotland Yard spokesman said: "The street was closed off for three hours while we were trying to discover the source of the odour."