Britain’s state-run health care service warned on Saturday that devices used by people with Covid-19 to monitor blood-oxygen levels at home may give inaccurate readings for people with darker skin.

The warning concerned pulse oximeters, currently being used by many of those at risk of severe Covid-19 symptoms to check their blood-oxygen levels. Below a certain reading, they need to be hospitalised.

The National Health Service, the UK state-funded health service, supplies them to those with virus symptoms, aged over 65 or clinically vulnerable.

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.

Updated guidance for virus sufferers on the main NHS website now warns: “There have been some reports they may be less accurate if you have brown or black skin.

“They may show readings higher than the level of oxygen in your blood.”

But the important thing is to check regularly whether to see if they are going down, it adds.

Habib Naqvi, director of the NHS Race and Health Observatory, said the issue affected “black (and) Asian diverse communities”.

Members of ethnic minorities, particularly Black Africans and Bangladeshi, have suffered the highest death rates from the virus in Britain.

The death toll from Covid-19 in Britain stood at 129,583 Saturday, one of the highest in the world.