Legionnaires' disease, the lethal bacterial lung infection that kills more than one in 10 of those that it infects, is spreading rapidly across Britain.

Record levels of the infection were recorded in five of the first six months of this year and experts say the worst is still to come.

The Health Protection Agency, which released the figures yesterday, said it was investigating the rise. The disease is caught when infected water is inhaled as a vapour and causes pneumonia-like symptoms which can lead to death.

Hotel showers and office air-conditioning systems are a particular risk. The latest outbreak occurred on the cruise ship Black Watch of the Fred Olsen line which was forced to return to Dover from the Baltic after two passengers contracted the infection.

The HPA figures show there were 163 cases up to the end of June, a third higher than the same period of 2006 (120 cases) and 60 per cent up on 2005 (103 cases).

Legionnaires' disease rises in spring and summer with the advent of warmer weather and peaks in August and September. Last year, total recorded cases of 559 were the highest since records began in 1980 and experts said the increase could be the result of climate change.

The summer of 2006 saw the hottest July on record and a wetter than normal August which provided the ideal breeding conditions for legionella bacteria.

In Health Protection Report, published yesterday, the Agency said: "Over 200 cases occurred in August and September and are being investigated for links to the warm weather experienced in 2006 and possible climate change effects on the ecology of the disease."

The weather since June this year has been very different to the previous two summers so it is "not clear" whether high numbers will be repeated this month and next, the report says.

However, the theory that climate change is driving the disease is borne out by the rapid growth in community-acquired infections which for the first time in 2007 have accounted for more than half (57 per cent) of the total. The remainder were in travellers and in hospital patients.

The proportion of community infections has grown from 41 per cent in 2005 and 47 per cent in 2006.

In past years, peaks had been linked to single outbreaks, such as in Barrow in Furness in 2002 when 179 people were infected by a leaking cooling tower.

Last year was the first time the annual total of cases exceeded 500. The total has exceeded 300 every year since 2002, having remained below 200 for the previous 20 years. Improved reporting has contributed to the rise, the HPA said.

Legionnaires' disease can strike at any age though it is more common in people over the age of 50.