Thousands of people who suffered severe allergic reactions after sitting on their sofas were victims of a toxic gas emitted by an anti-mould agent, a study has concluded.

Hospitals across northern Europe have treated thousands of patients with symptoms which appeared to range from skin cancer and chemical burns to severe eczema.

The British cases have been linked to an estimated 100,000 sofas sold by Argos, World of Leather and Walmsley Furnishing manufactured in China by a company called Linkwise.

A study in Sweden has concluded that the skin conditions were a reaction to the gas created during the sublimation of a chemical called dimethyl fumarate. Sublimation is the conversion of a solid to a gas without passing through a liquid state.

Sachets containing crystals of the chemical were placed between the cover and foam of the sofas to prevent mould developing during shipping.

More than 1,500 victims — including the relatives of two people who allegedly died as a result of their symptoms — have lodged compensation claims with British lawyers.

Relatives of Elizabeth McLaughlin, 59, claim that she fell gravely ill after using an Argos Bari sofa in last May and died two months later of heart failure.

Professor Magnus Bruze, a dermatologist from Malmö University Hospital in Sweden, said his tests had proved conclusively that the allergic reactions were caused by the dimethyl fumarate.

Professor Bruze added that it could take weeks or months to become hypersensitised to the chemical which disguised the link to the furniture in many cases.

Once a person is sensitised, the condition remains for life and could expose them to reactions to other chemicals.

“Some of the cases have been very severe and have been difficult to differentiate from the malignant cases of skin diseases like lymphoma, or a skin infection or chemical burn,” Professor Bruze said.

Experts had been left confused by the causes of the symptoms because the chemical sachets were concealed within the sofas and gas is a highly unusual cause of dermatitis. The gas was able to pass through clothing.

People who believe that they have been affected by the chemical can now take a “patch test” on a small part of their skin to check if they have been affected.

It is feared that contamination could have spread from the sofas throughout homes when the gas cools and re-solidifies.

Tests have concluded that the rate of sublimation increases with heat — such as the weather and the body mass of people using the furniture. This could mean a spike in cases this summer and that fatter people are more likely to be affected.

A judge last month gave approval for the group litigation against Argos, Land of Leather and Walmsley Furnishing. Mr Justice Treacy made a Group Litigation Order at Nottingham Crown Court for claims in what is thought to be the largest such action in Britain for a faulty consumer good.

Richard Langton, partner at solicitors Russell Jones & Walker, said it had estimated that more than 100,000 Linkwise sofas have been sold in Britain and compensation claims have already been lodged by 1,500 people.

“The long-term effects are not yet known but we’ve had reports of people being hospitalised and two alleged fatalities,” he said. “In many cases the pain is like the worst sort of sunburn or scalding form acid or boiling water. It has lasted for months or years if people don’t realise the cause.”

Mr Langton estimated around £6-10 million could be owed in compensation if all claimants were successful in the case.

After the ruling a spokeswoman for Argos apologised to any customers who may have been affected by skin irritations linked to sofas manufactured by Linkwise.

“We are pleased that the court has granted the Group Litigation Order today, which will help with the management and speedy resolution of the claims,” she said.

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