Hundreds of French nudists are threatening to "bare all" against burkini-clad bathers in an unlikely showdown at public baths in Grenoble as an ongoing row over the body-covering garment has resurfaced in the heatwave.

The city in southeastern France - which is sweltering in temperatures of over 40 degrees Celsius - obliges women to wear one-piece swimming costumes close to the body in swimming pools while men must wear Speedo-style trunks rather than shorts, for hygiene and security reasons.

But in recent days, a group of Muslim women called Citizen’s Alliance of Grenoble has twice turned up to pools in the city at the foot of the French Alps wearing the burkini.

In their first attempt, around 15 women managed to enter the pool on May 17 and filmed themselves bathing, saying they had done so "to defend freedom of religion”. Last Sunday, a group were fined €35 (£30) each for doing so.

Undaunted, they have called on Facebook for a third “operation burkini” this Sunday, claiming they are the “Muslim Rosa Parks” of France, a nod to the US civil rights icon.