The Barbastella bat is rare in the UK (Image by BCT/Hugh Clark)

The Stour Valley Project (SVP) is planning to start work on Wednesday on the first of 10 pillboxes along the River Stour in Essex and Suffolk.

The bunkers are being turned into caves where bats can hibernate during the winter, in a �5,000 project.

Pillboxes run the length of the Stour, which in 1940 formed a key defensive line against an expected invasion.

Peter Ennis, the SVP's landscape and biodiversity officer, said the pillboxes should be a good place for bats to hibernate because they "keep a fairly constant temperature and in the winter act rather like a cave".

"In the winter bats need somewhere to hibernate where the temperature is constant and the humidity is high. There aren't a lot of caves in East Anglia and we don't really know where bats do hibernate," he said.

Checking temperature

"They are structures which, because they are either buried or very substantial, keep a fairly constant temperature and in the winter act rather like a cave."

The shelters will be fitted with steel doors to prevent people getting in and disturbing the hibernating bats and the gun ports will be bricked up to help keep the temperature inside just right.

Finally, the bunkers' smooth internal concrete walls will be fitted with purpose built bricks and blocks, filled with nooks and crannies for the sleepy bats to crawl into or hang from.

Mr Ennis said: "We are putting computerised data loggers into the pillboxes to see what the temperature and humidity are like and if we don't get what we want we can do modifications."