People in France are being warned to avoid eating poppy seed bread after tests found that it contained morphine and codeine which could cause intoxication, vomiting or nausea.

French health officials are investigating the so-far-unexplained presence of the drugs in poppy seed baguettes and ready-made sandwiches made with poppy seed bread.

Poppy seeds do not normally contain opiates and government investigators suspect that a batch of seeds supplied to bakeries could have been contaminated from the latex sap of the plant, which contains alkaloids.

The investigators are unsure how much of the popular sandwich bread may have been contaminated.

Jean-Claude Alvarez, head of the toxicology department at the Raymond-Poincaré hospital in Garches, near Paris, said a single sandwich made with poppy seed bread could contain as much as four mg of morphine, the equivalent of nearly half a tablet of morphine sulphate which is given to people suffering from cancer.

“I strongly advise people not to eat poppy seed bread until we tell them otherwise,” Dr Alvarez said. “The drugs we have found (in poppy seed bread) are only supposed to be used by people in severe pain, and then on top of that there is the risk of addiction.”

“We must identify the source of the contamination and the companies that were supplied with this batch of seeds,” he said.