Police say robbers made off with a necklace worth HK$20 million after threatening jewellery store staff with a knife in a lunchtime heist in Tsim Sha Tsui on Monday.

Chief Inspector Yau Yu-sing of the Yau Tsim Mong District crime squad said two men entered the shop in Canton Road at around noon, posing as customers looking to buy the jade necklace. When staff showed it to them, one of the pair pulled out a 50cm knife and threatened the employees before taking the necklace.

The pair are said to have fled in private car driven by a third man. The getaway vehicle was abandoned in Yau Ma Tei soon afterwards, with the three suspects escaping separately via Man Cheong Street, Man Yuen Street and Ferry Street.

Officers say the knife was found in the car.

Yau said the force was now looking for the trio, who are described as being between 30 and 40 years old and between 1.7 and 1.75 metres tall. Police say all of them are non-Chinese.

He noted that one of the robbers had a black scarf wrapped around his head.

Dozens of detectives were seen investigating at the Emperor Watch and Jewellery branch on Monday afternoon.

Yau appealed to anyone who witnessed the robbery to contact the police.