Can undeclared driver use a car in France?

The issue here is whether or not you genuinely only drive the car on an occasional basis.

According to insurance rules, everyone who drives the car on any kind of regular basis should be known to the insurer so they can adjust the premiums depending on the way the car is used.

If someone other than the main declared driver or drivers uses the car frequently, it changes the risks involved in the use of it.

If other people drive it, this should only be exceptional, with typical examples being because the main driver had been drinking or did not have their glasses.

If you declare an accident and say you were driving it only on an exceptional basis, the insurer might carry out investigations if there is a large pay-out involved and/or they have doubts about the truth of the situation.

They have investigators, often retired police officers or gendarmes, on their books who may visit and ask questions of neighbours, for example.