Details of how BBC news bulletins were used to send secret messages during World Wor II, when Britain and France went to war with Germany, have been released in archive recordings.
The 80-year-old recordings show messages were sent to resistance fighters and helped them target bombs. It was done through music at the end of the news bulletins.
University of Sussex professor David Hendy said the bulletins broadcast to Poland would be made to run deliberately short, allowing for messages to be broadcast.
"A secret messenger from the exiled Polish government would deliver to the BBC, under a code name Peter Peterkin, a record that should be played in that spare minute or so," Mr Hendy explained.
"The choice of music would send a message to resistance groups in Poland."
A man named Alec Sutherland was responsible for getting the BBC to play the right tracks at the right time, even if they were scratched.
"The recorded programmes assistants would look at these discs and they would see a band which they thought would make a better broadcast, and so they would play the other broadcast and the wrong bridge would get blown up in Poland," Mr Sutherland said in a recording obtained by the BBC.
The recordings give new insight into the role broadcast played during the war.