Almost half Britain's horse chestnut trees could be infected with deadly bacteria, according to a new study that warns the disease has spread much further than experts realised.

A survey of more than 2,600 horse chestnuts across the country found that 49% showed symptoms of the bleeding canker disease, which attacks bark and can kill the tree or require it to be chopped down. Previous estimates said only 5% of trees were affected.

Roddie Burgess, head of plant health at the Forestry Commission, which carried out the new survey, said: "This was the first opportunity we have had to carry out a survey of this type, and the results did take us by surprise."

Commission staff checked the state of 1,385 rural trees and 1,244 in urban locations across England, Scotland and Wales last summer. More than half (54%) of the urban horse chestnuts showed symptoms of the disease and 44% of the rural trees were affected. The symptoms were most common in south-east England, where more than three-quarters (76%) of the trees surveyed showed symptoms. There are thought to be 1 million horse chestnuts in Britain.

The experts cannot be sure whether the sickly trees have the disease until they are screened for the bacterium responsible, Pseudomonas syringae

The emergence of the bacterium has baffled scientists, because for decades the bleeding canker disease was thought to be caused by a fungus. Joan Webber, a senior research scientist at the Forestry Commission, said: "It attacks the bark, kills it and makes it like a sore."

Several thousand horse chestnuts are thought to have been killed or removed after catching the disease, but Webber said: "We don't know how many trees succumb. It's not necessarily a death sentence for trees that become infected."

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