On the night of July 4, 1943, transport aircraft Liberator AL523 took off from Gibraltar's North Front tarmac and within minutes crashed into the sea with only one survivor, the pilot. The commander-in-chief of the Polish army and prime minister of the Polish government in exile, General Wladyslaw Sikorski, was dead.

Rumours as to the cause of the crash abounded. Was it pilot error? Was it, as officially classified, merely an accident, or was it, as the authors conclude in this riveting and meticulous study, an act of sabotage?

The catastrophe has led to several conspiracy theories over the years that still persist, often proposing that it was an assassination, variously blamed on the British, Soviets and Nazis. However, by sifting through numerous primary sources, the complete court of inquiry transcript, detailed analysis of aircraft components and systems, and unearthing many little-known eyewitness accounts, the authors' conclusion is compelling.

Despite AL523 being heavy and possibly overloaded, this was not the contributing cause of the crash. The pilot had competently taken off and cleared the runway. The sole reason for the aircraft's fall to earth was simply a cotton rag used by Polish saboteur Bronislaw Urbanski to obstruct elevator travel. He and he alone was responsible for one of the most shocking events of WWII.

This is an exhaustive piece of investigative journalism to put the record straight once and for all.