French police have opened an inquiry into allegations that a Russian human rights lawyer may have been deliberately poisoned in Strasbourg on Monday.

Karina Moskalenko, who represents some of the Kremlin's best-known critics, became ill after finding a substance similar to mercury inside her car.

French police said there had not been enough of the substance to endanger life, but that more tests were being carried out. Ms Moskalenko and members of her family were treated for nausea and headaches.

The lawyer's clients include the jailed former Russian oil tycoon, Mikhail Khodorkovsky, and the family of the murdered journalist, Anna Politkovskaya. Her illness meant that she was unable to travel to Moscow on Wednesday for a preliminary hearing at the trial of three men charged with Ms Politkovskaya's murder.

"People do not put mercury in your car to improve your health," she told a Russian radio station. "The question that preoccupies me now is: was it a demonstration, which I only noticed too late, or on the contrary, did I notice it too soon, and the plan was that it should travel with us for longer?"

Symptoms of mercury poisoning can include memory loss and mood instability. High levels of exposure can be fatal, especially if it is inhaled in vapour form.