On 4 July 1943, a Liberator II aircraft crashed off Gibraltar shortly after takeoff, killing all but one of the seventeen people on board.

The crash was ruled to have been an accident, but the conclusion sparked controversy on Sikorski's death, with several alternative theories put forward.

[1] To boost morale, Sikorski began a tour of inspection of the Polish forces stationed in the Middle East in May 1943, tending to political affairs where necessary.

[citation needed] In 1972, the pilot, Flight Lieutenant Eduard Prchal, described the events: "I received the green light from the tower and we began our take-off run.

[13] On 25 July 1943 the court concluded that the accident was caused by the "jamming of elevator controls" which led to the aircraft being uncontrollable after take-off.

[15] Slessor was not satisfied with the report and on 28 July ordered the court to continue its investigation to find out whether the controls were indeed jammed or not, and if they were, then for what reason.

"The political context of the event, coupled with a variety of curious circumstances, immediately gave rise to speculation that Sikorski's death had not been an accident, and might have been the direct result of a Soviet, British, or even Polish conspiracy.

Sikorski was exhumed and his remains were examined by Polish court experts, who concluded in 2009 that he died of multiple injuries consistent with an air crash, and possibly of drowning as an additional cause.

Finally, the autopilot was switched on just after the take-off – contrary to the flight manual – and that caused an effect similar to the controls' jamming as seen by the second pilot.

[27] Zięborak rejects General Nöel Mason-MacFarlane's opinion that Prchal's mental state during the take-off was the reason for the accident.

The reason for these differences, i.e. whether Prchal lied deliberately in his article or suffered from a type of partial amnesia as a result of his injury is not discussed.

The author concluded that this was a convenient solution for both the British and Polish government, as the details of VIPs' flight procedure could not be published in the Court's report during the war.

He agreed that the aircraft was overloaded and its take-off weight significantly exceeded the limit set by the manufacturer and RAF Transport Command.

Violation of weight regulations was, however, tolerated due to wartime transportation difficulties, and pilots were encouraged to take responsibility for flights with overloaded aircraft.

[32] Moreover, minor smuggling of scarce goods by flying personnel was widespread and the baggage of important passengers was not checked nor weighed at all.

Moroever, the main problem at this airport was bad weather conditions due to the mountainous environment, sea influences and winds, and accidents often occurred, including two involving other Liberators in 1942 and 1943.

Sikorski planned to use his trust in the Czechoslovak pilot for propaganda, to improve harsh relations between the Polish and Czechoslovakian governments in exile.

[33] After taking off too early, the aircraft started to lose height, and Prchal may have had an impression of jammed elevators due to a gust of wind.

[32] According to Różycki, a primary goal of the British Court of Inquiry was to investigate the possibility of sabotage in Gibraltar, which was of vital importance to other allied commanders and politicians.

In the opinion of Różycki, the final conclusion (that the accident was caused by the jamming of elevator controls of unknown cause) was deliberate understatement, chosen to avoid straining Polish relations with Czechoslovakia by blaming the pilot - and to avoid revealing negligence in transport pilot training and procedures, for which the RAF was responsible.