Mortuary workers were exposed to “dangerously high” levels of toxic substances from preserved bodies repatriated to the UK following the Air India plane crash, a coroner’s report has revealed.

Air India Flight 171 to London Gatwick struck a building shortly after take-off in Ahmedabad in June, killing 241 people on board and a further 19 on the ground.

Fifty-three British nationals were among those who died in the crash – which was survived by just one passenger.

A coroner’s report, released ahead of inquests taking place, said bodies repatriated to Westminster Public Mortuary, in London, had been wrapped and saturated in high concentrations of formalin, and laid in lined coffins.

Formalin contains formaldehyde and is commonly used to preserve human remains, especially for repatriations from abroad. However, it “became apparent that many of the mortuary users appeared unaware and were surprised by the nature of the danger”, the coroner said.

The substance has toxic effects at high levels and can cause severe respiratory irritation. There is also evidence long-term exposure can cause cancer, according to the UK Health Security Agency.

There is nothing in the report, known as a prevention of future deaths report, that says workers have been ill. It is also not clear who was responsible for the levels of formalin used.

However, senior coroner Fiona J Wilcox, for inner west London, said mortuaries needed to be better prepared and equipped.

“There is an under appreciation across mortuaries of the dangers posed by formalin to the health of all mortuary users,” she said. Mortuaries frequently receive bodies preserved in formalin, she continued, but it is “not routinely monitored”.

This means appropriate equipment “may not be available nor used when mortuaries handle bodies significantly contaminated with formalin, thus exposing users of mortuaries to health risks including risk of death”, the coroner added.

After the Air India repatriation, it became apparent there was “a significant chemical hazard” when mortuary workers were opening coffins, the coroner said.

“It is apparently not usual for environmental monitoring to be routinely available in either public or hospital mortuaries.”

The coroner found levels of formalin “were found to be dangerously high”, at “apparently 40%”.

“With heat and light exposure it breaks down releasing carbon monoxide which is highly toxic,” she wrote in the report. “If it mixes with a source of ammonia (commonly seen with decomposition), cyanide which is also highly toxic can be released.”

Sky News science correspondent Thomas Moore says it is fairly standard for repatriated bodies to be embalmed with formalin, and for some countries it is a legal requirement for infection control.

“But a concentration of ‘40%’ would be unusually high, as far as I can tell,” he said.

“Formalin is a 37% solution of formaldehyde and is carcinogenic with prolonged exposure. But it’s normally diluted before embalming, so the tissue concentration would be a lot lower.”

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Both carbon monoxide and cyanide were also detected in the mortuary “at dangerous levels following opening of the coffins and unwrapping of the bodies of the deceased persons”, the report said.

The report has been sent to the Department of Health and Social Care, and Ministry of Housing, Communities and Local Government, with the coroner saying action should be taken to prevent potential deaths in the future. They have 56 days to respond.

Sky News has contacted both departments for comment.