The UK’s Medicines and Healthcare products Regulatory Agency (MHRA) has warned that a selector switch for Philips’ HeartStart XL defibrillator could fail.

The agency said that the device should no longer be used because its therapy-selector switch could prevent proper activation. Philips discontinued sales of the product in December 2013, but some of the units are still in service.

The device may fail to turn on or may deliver a shock at a different energy level than selected, the agency said.