The city of Grenoble has gone to France's highest administrative court to challenge a ban imposed on a new city rule allowing full-body "burkini" swimsuits in public pools.
Grenoble's decision to authorise all swimwear, including burkinis, sparked a legal battle with the government.
Burkinis are worn largely by Muslim women, as a way of preserving modesty and upholding their faith.
But religious expression in public life in France can be divisive.
Ahead of Tuesday's court case, Interior Minister Gérald Darmanin described Grenoble city council's swimwear policy as an "unacceptable provocation" that was contrary to French secular values. Last month, a local court in Grenoble suspended the policy on the grounds that it seriously undermined the principle of neutrality in public services.
The ban on burkinis in state-run pools is also advocated for reasons of hygiene. Men are normally obliged to wear tight-fitting swimming trunks - another rule that Grenoble has decided to overturn. The city council has also permitted men to use Bermuda shorts, which are not usually allowed.
What is a burkini?
- A burkini is a full-body swimsuit that covers everything except the face, hands and feet
- The name is a mix of the words "burka" and "bikini"
- Unlike burkas, burkinis leave the face free
- Burkinis are marketed to Muslim women as a way for them to swim in public while adhering to strict modesty edicts
French opposition to the burkini stretches as far back as 2016, when several local municipalities attempted to outlaw it on beaches for violating the country's strict separation between religion and the state.
State officials are not allowed to wear religious emblems at work, but the mayor of Grenoble, Eric Piolle, argues that this should not stop users of public services, such as swimming pools, from dressing as they wish.
His administration's moves to relax the swimwear rules are opposed by the national government, which has invoked a law passed last year to combat "Islamist separatism".
The row has now reached the Council of State, which has previously ruled against a burkini ban elsewhere in France.
Critics of the burkini see it as offering a separatist vision of French society and argue that allowing it also puts pressure on Muslim women to wear it. Far-right leader Marine Le Pen has condemned the burkini as "clothing of Islamist propaganda".
However, those in favour of allowing it say women should have the choice to keep their bodies covered if they want to and that this does not imply religious extremism.
The court is due to deliver its ruling in the coming days.