Staff, patients and visitors are being asked to wear masks on the wards at three hospitals as flu cases rise.
The measure has been introduced 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 flu patients had doubled in the past two weeks.
It said there had been outbreaks on the wards and the dominant strain was “particularly virulent”.
Respiratory consultant at Frimley Health NHS Foundation Trust, Dr Gareth Roberts, said: “We always expect an increase in flu cases at this time of year, but over the last couple of weeks it has become a more serious problem.
“We ask all of our staff and patients to wear masks while on hospital grounds.
“The steps we are taking are not easy, but it is important to us to be able to protect our staff and patients.
“It’s important that we make sure our employees are in good shape.”
The UK flu season is earlier than usual and has not yet reached its peak, the government has previously warned.
According to the UK's Health Security Agency, the rate of flu-related hospitalizations in the last week of November was 63% higher than the week before.
Several hospitals found themselves in a difficult situation and reintroduced masks, including Guildford A&E Department.
Frimley Health NHS Trust said it would supply masks to clinical ward staff and visitors at hospital entrances.
Additional hand gel dispensers will also be provided, the foundation said.
It says the current H3N2 flu strain can cause severe symptoms within a week, even in some patients who are already vaccinated.
