This photo is used for illustrative purpose.

Face masks can become less effective when worn in the rain, the World Health Organisation (WHO) and scientists have warned.

The health group and a number of experts have stated that face coverings must be replaced if they become damp and urged government officials to provide the public with “clear advice” about wearing masks during wet weather.

While it is not compulsory to wear protective face coverings when outdoors in the UK, some people opt to keep their masks on while walking between different sites.

Tim Spector, professor of genetic epidemiology at King’s College London, told The Times: “It would now be useful if clear advice were issued to the public.

“Masks need to be changed regularly and this is particularly important to understand in damp and wet weather.”

Aseem Malhotra, a consultant cardiologist, added: “It is obvious that masks will get damp as people shop and travel in bad weather.

“There has been no public campaign to make people aware that this can make their masks ineffective.”

According to official guidance from the Department of Health and Social Care, the public should "change the face covering if it becomes damp or if you’ve touched it."

Similarly, advice from the WHO states that fabric face masks should consist of at least three layers of different material. However, it adds that moisture can compromise this protection.

It adds that while N-95 masks are considered among some of the most effective in preventing infection, the non-woven fabric used does not repel water, meaning that any moisture can restrict safe and efficient filtering.

“For any type of mask, appropriate use and disposal are essential to ensure that they are as effective as possible and to avoid any increase in transmission,” the WHO states.

“All masks should be changed if wet or visibly soiled; a wet mask should not be worn for an extended period of time. Replace masks as soon as they become damp with a new clean, dry mask.”

Karol Sikora, a former chief of the WHO’s cancer programme, said: “Moisture makes masks porous and because of this all types of mask are essentially vulnerable in damp weather.

He added that the public need to “be given clear advice by the authorities”, particularly given the recent bout of wet weather across the UK.

The comments come after Boris Johnson announced new rules on face coverings, increasing the number of places in England where it is mandatory to wear a covering.

People were already required to wear a face covering in shops and supermarkets, as well as on public transport. However, under the new rules people are now also expected to wear them in taxis, private hire vehicles, and hospitality venues when customers are not eating or drinking.

Johnson said the country had reached a “perilous turning point” and the government was now acting on the “principle that a stitch in time saves nine”.