A new theory on the mysterious disappearance of Malaysia Airlines Flight MH370 has pointed to 221kg of lithium ion batteries on board.

US pilot and aviation engineer Bruce Robertson speculated on his website that the cargo of batteries caught fire during the flight, sending a cloud of deadly carbon monoxide into the cabin.

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He described what might have happened next.

“Pilot Zaharie Ahmad Shah falls victim to the fumes, but co-pilot Fariq Abdul Hamid turns the plane back to the west and begins a descent with the intent of landing,” Mr Robertson wrote in the report.

“Fariq succumbs to the carbon monoxide and the plane’s automation takes over the next several hours.

“As the plane is blind to the world (and the world is blind to the plane), the plane flies in a very large radius left turn, the exponential spiral path first proposed in March 2014.

“The plane crashes into the Southern Indian Ocean west of the Zenith Plateau, west of Exmouth Australia. This is at roughly 21 degrees south, 103 degrees east.”

Mr Robertson believed the Boeing 777 came down much further north of the site where searchers were working.

“Too much time and money has been wasted on a fruitless search in an area much further southwest, due west of Perth,” he said in the report.

In the latest of a string of theories, earlier in June, a team of mathematicians said the Boeing 777 vanished without a trace because it plunged into the Indian Ocean at a 90-degree angle.

The aircraft, carrying 239 people, mysteriously disappeared from radar and stopped transmitting its position on March 8, 2014, with searchers suspecting it flew on for six more hours before meeting its fate over the southern Indian Ocean.

The search, which has involved ships dragging submerged sonar bouys along narrow paths of a 60,000 square kilometre search area, has been doubled to 120,000 square kilometres. So far 48,000 square kilometres has been covered, the ATSB reported.