The hotel's leisure centre is thought to be the source of the outbreak

Doctors have treated three people with the disease, which has been traced back to a leisure centre at a local hotel.

Experts say the risk of infection at the Cricket St Thomas Hotel in Chard has been isolated.

But they are warning anyone who has used the hotel's facilities in the last month to watch for symptoms.

One of the people diagnosed has made a full recovery, but two others are still in hospital.

Contaminated water vapour causes the disease

The leisure centre's showers and spa tub have been closed for extra cleaning, but its pool, which is chlorinated, is open.

It is the second time in recent months that the hotel, which was the setting for BBC television series To The Manor Born, has been at the centre of a health scare.

Dozens of guests were affected by the Norwalk virus, which causes vomiting and stomach upsets, before Christmas - forcing it to close for three days.

"We understand that people might be concerned about hygiene," said hotel spokeswoman Sarah Gallo.

"But it is important to note that Norwalk Virus is an airborne virus which was brought into the hotel by someone visiting the hotel.

"What we are dealing with now is a bacteria, but we are dealing with it in all the ways outlined by the Somerset Health Protection Unit."

The outbreak is thought to centre on the hotel's spa

Legionnaires' disease is transmitted by airborne mist particles from contaminated water sources, such as whirlpool baths and air conditioning units.

Symptoms of the illness, which is not transmitted from person to person, include a chest infection, fever, chills and a cough.

Its incubation period is between two to 10 days and up to one in seven cases can be fatal.

Experts are warning anyone who has stayed at the hotel since the beginning of the year who suspect they may have the disease to contact their GP for tests.

Dr Elaine Farmery of the Somerset Health Protection Unit said: "We are quite concerned that there are people out there - all over the country - who may not be taking the appropriate antibiotics.

"They may just think they have the flu, but normal antibiotics will not treat the legionella organism."