Karina Moskalenko, 54, who is also a lawyer for former Yukos owner Mikhail Khodorkovsky, said in an interview with Ekho Moskvy radio that Strasbourg police had found a huge amount of a substance similar to mercury in her car.
Mercury is a highly toxic metallic element that is a liquid at room temperature.
The discovery was made just before hearings Wednesday by a Moscow military court into Politkovskaya's murder case. An appeal against a ruling to deny Khodorkovsky parole will be considered in Siberia's Chita on the same day.
Politkovskaya, who gained international recognition for her criticism of the Kremlin and reports of military atrocities against civilians in the troubled Caucasus republic of Chechnya, was gunned down in an elevator in her Moscow apartment building in October 2006.
Moskalenko, known for having defeated the Russian state at the European Court of Human Rights in Strasbourg, and members of her family showed symptoms of mercury poisoning and were taken to a hospital. Tests to identify the substance are underway.
"The doctors are not releasing [Moskalenko], she will be unable to participate in tomorrow's hearings into Politkovskaya's murder case for health reasons," lawyer Anna Stavitskaya, who also represents Politkovskaya's family in court, said Tuesday, adding that the defense had asked the court to adjourn Wednesday's hearings.
Russia's Prosecutor-General's Office has sought to stop Moskalenko acting as Khodorkovsky's lawyer, accusing her of negligence in defending her client. Khodorkovsky, currently serving an eight-year prison term for fraud and tax evasion, has made no complaint.