A French doctor already charged with poisoning seven patients was detained on Tuesday for questioning over suspicions that he poisoned a further 50.

Frederic Pechier worked as an anaesthesiologist at two private clinics in Besancon, eastern France.

Seven patients, aged 37 to 53, went into cardiac arrest from 2008 to 2017. Five were saved but two died. Investigators later found traces of potassium in their bloodstreams five times the lethal dose.

Pechier, who was first charged in May 2017, has denied the allegations and was released pending trial but banned from working, despite his request to continue acting as a consultant while not being allowed to enter operating theatres.

Police have since widened the net in their two-year investigation. In January, four deceased former patients - two women and two men - were exhumed and tests conducted for any high traces of potassium.

“This new detention comes as part of a preliminary inquiry into cases of poisoning that may have been committed at the Saint-Vincent clinic in Besançon,” said local prosecutor Etienne Manteaux.

According to judicial sources cited by AFP, police have homed in on "around 50 reports of unwanted events" - a medical term referring to the advent of complications, a critical condition or death among otherwise healthy patients.