Staff, patients and visitors are being asked to wear face masks on wards at three hospital sites as flu cases rise.

The measure has been brought in at Wexham Park Hospital in Slough, Heatherwood Hospital in Ascot, Berkshire and Frimley Park Hospital near Camberley, Surrey.

Frimley Health NHS Foundation Trust, which runs the sites, said the number of patients with flu had doubled in the last two weeks.

It said there had been outbreaks on wards and the dominant strain was “particularly virulent”.

Respiratory consultant at Frimley Health NHS Foundation Trust, Dr Gareth Roberts said: “We are always expecting an increase in flu numbers at this time of year but it has become more of an issue over the last couple of weeks.

“Over the hospital sites we are asking all of our staff and patients to be wearing masks.

“The steps that we are taking are not something that we take lightly but it is very important for us to be able to protect our staff and patients.

“It is vital that we ensure our staff are fit and well.”

The flu season has arrived earlier than normal in the UK and has not yet peaked, the government has previously warned.

The rate of flu hospitalisations in the last week of November was 63% higher than the week before, according to the UK Health Security Agency.

Several hospitals have been put under strain and have reintroduced masks, including at Guildford’s accident and emergency department.

Frimley Health NHS Trust said it would supply masks for staff on clinical wards and for visitors at the hospital entrances.

Extra hand gel dispensers will also be provided, the trust said.

It said the current H3N2 strain of flu could cause severe symptoms for a week, even for some patients who are already vaccinated.