A Saharan dust cloud is expected to settle over parts of the UK on Thursday, prompting health experts to issue warnings to elderly people and those who suffer from breathing problems.

High levels of pollution are expected over southern and central England, with Lincolnshire bearing the brunt of the cloud.

A yellow "moderate" warning has been issued for Merseyside.

The Department for Environment, Food and Rural Affairs issued alerts for most of England and Wales, warning that older people, asthma sufferers and those with breathing difficulties are most at risk. People are advised to avoid exercise if they notice problems.

Defra said: “Adults and children with lung problems, and adults with heart problems, should reduce strenuous physical exertion, particularly outdoors, and particularly if they experience symptoms.

“People with asthma may find they need to use their reliever inhaler more often. Older people should also reduce physical exertion.”

Asthma UK warned of conditions to cause problems for sufferers, with temperatures hovering around 17C.

Andrew Proctor, director of advice and support, said: “People with asthma need to take extra care. We are facing a perfect storm of asthma triggers, with air pollution and weather changes increasing the risk of potentially fatal asthma attacks for millions of people with the condition.

“Two-thirds of people with asthma find that air pollution makes their asthma worse, and even more tell us that changes in the winter weather can trigger asthma attacks.

“If air pollution or cold weather makes your asthma worse, do check forecasts and carry your reliever inhaler with you at all times.”