The conversation included boasts that a song would climb up the charts a veiled reference to increases in jihadi recruits once the attacks had been launched.
It is believed that at least one of the airlines involved had already been placed on a UK aviation watchlist.
GETTY London was one of the terrorist’s targetsLast night a senior intelligence source said: The areas we believed these attacks might take place were given extra surveillance.
The voices of both pilots were found to be clean after being passed through a database of potential terrorists.
Suspicions were raised and the recordings sent to GCHQ in Cheltenham, where analysts took seven hours to decode the transmissions.