The use of the unusual ingredient - used to make opium - at the restaurant in Yan'an, in Shaanxi province, came to light after one of its clients tested positive in a routine urine test by traffic police, despite insisting he'd never touched drugs, the Xi'an Evening News reports.

Suspecting the noodle shop he'd eaten at a few hours before the test might be to blame, the customer, Liu Juyou, persuaded relatives to frequent the shop as well and submit themselves to drugs tests. They also tested positive.

The shop owner - named only as Zhang - has since admitted to police that he bought 2kg (4.4lb) of poppy buds - which contain the plant's seeds - for $100 (£60) last month, crushed them into a powder and started to add that to his noodles, the Hua Shang Bao daily reports.

Police said the unprocessed seeds contain enough opiates to gradually build up in the body and eventually trigger a positive drugs test result.

According to the South China Morning Post, poppy seeds used to be a popular ingredient in Chinese hot pot sauce until their use was banned.