Photo: shaped like an amphitheatre to focus sound, the 200 feet long acoustic 'mirror' wall is one of only two in the world. Photo: Chris Reeve.

Three massive concrete "listening ears" built on the Kent coast between the wars to detect aircraft crossing the channel, are to be rescued with a £500,000 grant from English Heritage.

Precursors to radar, the structures were built in the 1920s as part of the vital inter-war experiments with early warning systems that eventually helped win the Battle of Britain.

Rapidly sliding into flooded gravel pits at Greatstone, Kent the scheduled monuments have been in a crumbling state since the 1970s.

Restoration work is set to begin on August 4 and will provide controlled access and interpretation for history enthusiasts and the general public.

Photo: over 30 feet high, the listening posts are precursors to radar and were part of vital inter-war experiments that eventually helped to win the Battle of Britain. Photo: Chris Reeve.

English Heritage Inspector of Ancient Monuments for Kent, Peter Kendall explained why the bizarre concrete structures need to be rescued.

"Standing like huge modern sculptures in the otherwise featureless gravel, these structures are beautiful and fascinating," he said, "as well as historically important for the critical role they were designed to have defending England,"

The posts consist of two huge bowls, 20 and 30 feet high, and a 200 feet long acoustic 'mirror' wall, shaped like an amphitheatre to focus sound – one of only two such structures in the world.

Forming part of a long tradition of communications and defence on the south coast, the devices worked by concentrating sound waves onto microphones. Operators would then use stethoscopes attached to the dishes to listen for the far-off sound of enemy aircraft movements.

Photo: paid for by the Aggregates Levy Sustainability Fund, work to restore the structures is set to begin in August. Photo: Chris Reeve.

During the Second World War, radar very quickly rendered the structures obsolete and since then the land has been used by the aggregates industry.

A Site of Special Scientific Interest, it is hoped that when restoration work is completed in October – before migrating birds arrive for the winter - the RSPB will manage it as a nature reserve.

Raised through the Aggregates Levy Sustainability Fund, the £500,000 grant will be used to repair and underpin the structures. The grant will also be used to shore up the lake edges and cut the causeway currently leading to the listening posts; in future a retractable bridge will provide access.

An additional £125,000 has been secured by local authorities to provide visitors with interpretation as part of the Historic Fortifications Network, a project to promote historic military sites in Kent, Flanders and around Calais.