When a young BBC radio assistant during the Second World War was handed a scratched disc to play on air, they did the helpful thing: swapped it for a different one.

The music was simply there to fill short gaps between wartime news broadcasts, the assistant reasoned, so the tune itself was of little consequence.

Unfortunately, that was not the case. The first record was a coded message for resistance fighters in Europe, and by playing another the assistant was unwittingly hampering the war effort.

The are revealed in an archive of material that sets out in detail for the first time how the BBC sent secret messages across the airwaves on its European service.

A 1941 memo explains how a Lieutenant Peter Peterkin - a codename - would arrive at the BBC’s Bush House headquarters with a particular record under his arm.

“I have agreed to the following temporary arrangement for the inclusion of gramophone records in the 4pm and 9.30pm Polish bulletins daily,” a senior BBC executive wrote.

“A Polish officer giving the name of Peter Peterkin will come to Bush House each afternoon and evening at 3.45pm and 9.15pm and will ask for you or your deputy. He will bring with him a gramophone record which he will hand over to you and he will wait until after the transmission to receive the record back.