A Chinese noodle shop owner was arrested for serving food laced with poppy seeds unbeknown to his customers, in the hope they would develop a habit of returning.

The man, identified only as Zhang, admitted to police he added the secret ingredient used to make opium, in an effort to boost customer loyalty, the Xian Evening News reported.

Detectives became interested in the restaurant in Yan'an, in the central Shaanxi province and its tasty noodles, after a client tested positive to a routine police drug test.

Liu Juyou, 26, explained he was bewildered by the test result, as he had never touched illegal substances in his life.

Suspecting his favourite noodle shop might have been serving him something off the menu, Liu persuaded his family members to eat at the restaurant and subsequently take urine tests - to which they tested positive.

Confronted by anti-drug agents, Zhang confessed he had bought some 2kg of poppy buds for 600 yuan (£60), crushed the seeds, obtaining the "secret" powder for his noodles.

Authorities said that opiates contained in the seeds can build up in the body due to regular use and eventually result in a positive drug test, the South China Morning Post reported.

Poppy seeds are a popular ingredient for recipes of different cuisines around the world but their use is banned in China.

Zhang's special recipe cost him 10 days behind bars.