Twenty-five people go on trial in southern France on Monday accused of butchering horses from the pharmaceutical industry and selling their meat even though it was unfit for human consumption.

The main defendant on trial in the city of Marseille is a meat wholesaler, Patrick Rochette, but other people, including vets, dealers and butchers also face charges over what investigators called "large-scale fraudulent practices."

The alleged scam saw fake documentation given to horses that had been previously used to produce serums by French pharmaceutical company Sanofi-Pasteur at a laboratory farm in southern France.

Even though their documentation identified them as horses "permanently barred from slaughter for human consumption", they were given new paperwork and butchered in Narbonne in southern France, Verona in Italy and Barcelona in Spain.  

However, an expert tasked by investigators to establish the possible danger of eating their meat concluded that there was "no major toxicological risk" for consumers.

Due to their large blood and urine-producing capabilities, horses have long been used in the pharmaceutical industry to produce vaccines, serums and other therapeutic products.

Sanofi's animals, former racing horses which investigators found had cysts and ganglions on their necks from repeated injections, had been destined for retirement after being released by the company.

Horsemeat is typically cheaper than beef and has long been part of diets across European countries, including France, but its popularity has waned in recent decades, and its production is strictly regulated.

The biggest horsemeat scandal erupted in 2013 when millions of ready-made meals were withdrawn from stores across Europe after they were found to contain horsemeat instead of only beef, as indicated on the label.

Meanwhile, verdicts are due Wednesday in the separate trial in Marseille of 18 people who are also accused of running a fraudulent horsemeat trading network.

French, Belgian and Dutch nationals have been on trial since June for violating EU sanitary rules governing the horsemeat trade, and with forging official documents between 2010 and 2015.

They are accused of duping the owners of ageing horses into believing that their animals would live out their days in the countryside when, in reality, they were taken straight to the slaughterhouse.