A Johnson & Johnson unit was accused of injuring hundreds of patients by selling “defective” hip implants, according to lawyers suing the company on the first day of a London trial.

Some of DePuy International Ltd.’s Pinnacle hip replacement parts cause metal particles to be released into the body, causing a range of health problems, lawyers for the 341 claimants said. The products were wrongly marketed as having low wear rates, according to the lawsuit. DePuy denies liability.

The case is one of the largest so-called product liability group actions to reach a British court, according to the claimants’ law firm Leigh Day. U.K. lawsuits have also been filed against every other company that manufactured “metal on metal” hip implants, and those cases have been delayed pending the results of the DePuy trial, the company’s lawyers said.

The first stage of the trial will deal with whether the J&J unit is responsible for the injuries. If the claim is successful, there will be further court hearings on the amount of compensation. Other lawsuits over Pinnacle artificial hips have been filed in the U.S. where, in January, a district judge in Dallas cut almost in half a $1.04 billion jury award against J&J.

DePuy’s lawyers said the claimants’ case was “not clear” and denied being liable for the injuries, according to their documents from the U.K. trial. “The burden of proving that the product was defective and that the defect caused harm rests on the claimants.”

DePuy said in a separate statement that the device in question “is backed by a strong record of clinical data showing reduced pain and restored mobility for patients suffering from chronic hip pain.” The company is “committed to the long-term defense of the allegations in this litigation.”