More than 1,000 runners have fallen ill after competing in a French mud race in which it was suspected animal faeces had become mixed with the mud and dirt.

Held on June 20, pictures taken at The Mud Day race showed participants crawling through puddles and over obstacles coated in dirt.

Initially only 30 people complained of diarrhoea and vomiting, but in the days following this figure skyrocketed to more than 1,000.

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A competitor from The Mud Day takes a tumble in the mud while participating in the gruelling event

A group of men stumble through a pool of mud as they attempt to complete the 8miles-long challenge

This year a record entry field of 8,000 people took part in the event, which was held on the French Riviera

After being flooded with complaints from ill participants, organisers promised to find out what caused the 'stomach upsets', 9News reported.

However, it is thought the illness may have been caused by animal manure that had mixed with the mud, which some participants claimed they could smell on the day.

The course itself stretches across 8miles and runners must cross electrified barbed wire, climb through muddy tunnels and over dunes.

This year a record entry field of 8,000 people took part in the event, held on the French Riviera, in Nice.