Footballer Emiliano Sala and his pilot David Ibbotson were exposed to 'harmful levels' of carbon monoxide when their plane crashed earlier this year.
The Air Accident Investigation Branch said the pair were exposed when the aircraft crashed in the English Channel on 21 January.
The AAIB's report states that toxicity analysis of Mr Sala's blood showed a carboxyhaemoglobin (COHb) saturation level of 58 per cent.
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According to the reports: "COHb is the combination product of carbon monoxide (CO) with haemoglobin, the oxygen-carrying protein molecule contained in red blood cells.
"A COHb level of 50 per cent or above in an otherwise healthy individual is generally considered to be potentially fatal.
"In this type of aircraft, the cockpit is not separated from the cabin and it is considered likely that the pilot would also have been affected to some extent by exposure to CO."
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It goes one: "Exposure to CO can lead to damage to the brain, heart and nervous system. The symptoms of CO poisoning worsen with an increasing percentage of COHb."
The symptoms listed for someone who has levels of 50 per cent of more include seizures, unconsciousness and heart attacks.
The body of Mr Ibbotson, from Crowle, Lincolnshire, has still not been found by authorities. However, as the report states, it is likely that he would have been exposed to the same or similar level of toxicity as the Argentine footballer.
The plane, which was travelling from Nantes - where Mr Sala's previous club is based - disappeared from radars on January 21.
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He was on his way to meet new teammates at Cardiff after signing a £15 million transfer deal from the French club.
Contact was lost while the plane was travelling at 2,300 feet, prompting a huge search to find both men.
The official search for the two men was called off on 24 January when Guernsey Police confirmed that they were 'no longer actively searching' for the plane.
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But the wreckage was eventually found after renowned shipwreck hunter David Mearns launched a survey vessel and discovered the aircraft on the seabed of the English Channel.
Recalling the discovery, Mearns told Sky Sports News: "Our job is done. This is an active investigation now in the hands of the AAIB (Air Accidents Investigation Branch), and they will be taking over as agreed.
"We've had a long trip back and all I could think about was the families. This is what they wanted us to do, we've achieved it and that's the best we could have hoped for - the best outcome.
"An incredibly tragic and sad, and a devastating moment, but at least some of their questions will be answered because of what we will be able to do today."
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