Britain's state-run healthcare service warned that devices used by people with Covid to monitor blood oxygen levels at home may give inaccurate readings for people with darker skin, AFP reported.

The warning concerned pulse oximeters, currently being used by many of those at risk of severe Covid symptoms to check their blood oxygen levels. Below a certain reading, they need to be hospitalised.

The NHS said in a statement that “there have been reports that pulse oximeters can be less accurate for people with darker skin because they may show higher readings of the oxygen level in the blood”.

The devices, clipped on to a finger, work by shining a light through a person's skin to measure the level of oxygen in the blood.