After dark - counting the bats in the exchange's roost

The pipistrelle bats were found when BT engineers went to inspect the Oak House building in Monmouth ahead of some modernisation work.

Now the telephone company is seeking help from the Countryside Council for Wales to ensure the protected species is not disturbed.

BT plan to construct a new access hole in the building for the bats to use.

The pipistrelle bat is the smallest and most widespread of Britain's 17 species of bat.

It measures up to 5cm (1.97in) from head to toe and weighs less than 5g (0.18oz).

The pipistrelle bat is one of the smallest of the British species

Jack Foxall, of BT Property said: "We are doing work to separate the old listed Oak House building in Priory Street with a newer extension at the back.

"The link between this and the old building at the front is where the bats are roosting behind the cladding on a wall.

"We are happy to share our accommodation with the bats though and we will be providing them with a new access hole.

"The important thing is that we do this work without causing them unnecessary disturbance," he added.

Erica Colkett, species officer for the Countryside Council for Wales, said: "This is quite a big colony - much bigger than you would find in the average roost site and quite important for the area.

"Although the pipistrelle bats are our most widespread species, numbers have declined dramatically since the 50s with only 40% of the population now that we had then.