More than 400 patients who signed up to take a pioneering oncology detection test developed by US biotech company Grail received erroneous letters last month suggesting they may have developed cancer.

According to an internal company document seen by the Financial Times, 408 patients were incorrectly told they had a signal in their blood suggesting they could have cancer.

Grail said the letters were sent “in error” by its telemedicine provider PWNHealth and that its staff had moved swiftly to contact affected customers to reassure them their test results were wrong.