France's top administrative court on Tuesday blocked a bid to allow the "burkini" at municipal pools in the city of Grenoble, upholding a government challenge against a move that revived France's intense debate on Islam.

The all-in-one swimsuit, used by some Muslim women to cover their bodies and hair while swimming, is a controversial issue in France where critics see it as a symbol of creeping Islamisation.

The "very selective exception to the rules to satisfy religious demands... risks affecting the proper functioning of public services and equal treatment of their users" the Council of State ruled Tuesday.

Its judgment backed a lower court ruling in May that was prompted by a challenge by the national government to the council's decision.

Led by Green party mayor Eric Piolle, Grenoble had in May changed its swimming pool rules to allow all types of bathing suits and for women to bathe topless.

Previously, only traditional swimming costumes for women and trunks for men were authorised.

"All we want is for women and men to be able to dress how they want," Piolle said at the time.

But the judges disagreed.

They ruled Tuesday that "contrary to the objective declared by the city of Grenoble, the change to the pool rules aimed only to authorise wearing of the 'burkini'".

The city council in Grenoble said in a statement Tuesday evening that it took note of the court ruling, while regretting that the court had attributed views to the council that it did not hold.

The changes it had tried to implement had been about allowing equal access to all users to public services, it argued.

The all-in-one swimsuit is used by some Muslim women to cover their bodies and hair while swimming
The all-in-one swimsuit is used by some Muslim women to cover their bodies and hair while swimming AFP / SAEED KHAN

Interior Minister Gerard Darmanin welcomed the ruling, describing it on Twitter as "a victory for the law against separatism, for secularism and beyond that, for the whole republic".

France's treatment of Islam is likely to remain a political battlefield following Sunday's inconclusive parliamentary election, which denied President Emmanuel Macron an absolute majority in parliament.

Voters handed the anti-immigration, anti-Islam National Rally an unprecedented 89 seats.

The more mainstream Republicans conservative party is seen as the only possible partner for Macron to reach a governing deal or pass laws on a case-by-case basis.

Republicans lawmaker Eric Ciotti called on Twitter Tuesday for the burkini to be "clearly banned by law".

Attempts by several local mayors in the south of France to ban the burkini on Mediterranean beaches in the summer of 2016 kicked off the first row around the bathing suit.

The restrictions were eventually overturned -- also by the Council of State -- for being discriminatory.

But Tuesday's ruling did not call that previous one into question, said Patrice Spinosi of the Human Rights League (LDH).

"This decision is only relevant to the specific situation in Grenoble and shouldn't be generalised," he said. The LDH had backed Grenoble's rule change over swimwear.

Burkinis are not banned in French state-run pools on religious grounds, but for hygiene reasons. Swimmers are not under any legal obligation to hide their religion while bathing.

Grenoble is not the first French city to change its rules.

The northwestern city of Rennes quietly updated its pool code in 2019 to allow burkinis and other types of swimwear.