A Paris judge on Monday asked two medical experts to determine whether rock star Johnny Hallyday was the victim of a botched operation that landed him in hospital with a severe infection.

Hallyday's lawyer described the decision as an "important step" in the case against Stephane Delajoux, dubbed the "surgeon to the stars", who performed the operation in Paris in November to correct a herniated disc.

An infection specialist and a neurosurgeon will take several months to study Hallyday's medical records before making a decision, lawyer Virginie Lapp said.