Lawyers acting for the six volunteers who suffered multiple organ failure during the trial of a new drug last month fear the men may not get full compensation because the company accepting liability for the injuries was underinsured, the Guardian has learned.

Martyn Day, solicitor for four of the men, said the German pharmaceutical company TeGenero, which created the drug TGN1412, had insurance of only £2m for the trial but the liability for damages and losses suffered by the men may be much bigger.

The revelation came as the full horrifying extent of the injuries suffered by one of the men emerged yesterday. Ryan Wilson, 20, the man who suffered the most severe effects of the drug, said he would lose some of his fingers and toes and would be in hospital for another six months. Mr Wilson has defied the expectation of doctors treating him at Northwick Park hospital, north London, after he survived heart, kidney and liver failure, pneumonia and septicaemia in the days after receiving the dose of TGN1412.

But his hands and feet are swollen and rotten. "Now they're waiting for my body to heal itself. I'm told it's like frostbite and my fingers will just fall off," he told the News of the World.

"From what I've been told, I'm not supposed to be here. When I came around from the sedation, most of the doctors had a shocked look on their faces and said, 'You're lucky to be here'," he said.

He said he only took part in the trials, for which the volunteers were paid £2,000, to buy driving lessons and a holiday and the only side effects he was warned of were headaches and nausea.

The long-term prognosis of all six men is uncertain but they have been warned they may have suffered serious damage to their immune systems. The drug was developed to treat auto-immune diseases such as rheumatoid arthritis, and had never been tested before in humans.

The consent form the men were asked to sign showed that TeGenero was aware the drug might seriously damage the immune system. It warned of the possibility of a "cytokine release", a massive immune system reaction.

Mr Day, senior partner in the law firm Leigh, Day & Co, said: "The consent form which each of the clients signed said that TeGenero would pay compensation as a matter of course. Now we find that in fact there may well not be enough money for the men to be paid what they need."

TeGenero was founded in June 2000 with €14.1m (£9.75m) in venture capital, he added. "This was the first drug the company had tried to produce, so there must be a real worry their backers will now walk away." That would leave the men with only the £2m insurance policy to rely on. "It seems unlikely that we would be able to persuade the courts that any other company, organisation or individual could be held liable for their injuries," he said.

Richard Powell, an expert in clinical immunology at Nottingham University, said £2m was unlikely to be enough to compensate the men for the long-term effects to their health. He said they could develop auto-immune disorders, in which the body attacks itself by mistake, the very type of condition the drug was being developed to treat.

A second risk, he added, was that the men's immune systems could be compromised forever, making them prone to infections or even tumours.

Senior TeGenero executives said last night the company agreed, before the start of the trial. to compensate the volunteers in the event of any injury, according to Association of the British Pharmaceutical Industry (ABPI) guidelines.

Benedicte Hatz, chief executive, and Thomas Hanke, chief scientific officer, said: "Based on information currently available, TeGenero has no reason to believe the volunteers will not be appropriately compensated."

They did not comment on the amount of insurance cover, but added: "In the context of the no-fault compensation scheme applicable to these claims, there is no need for the claimant's lawyers to incur substantial costs." Mr Day also questioned the role of the regulatory authorities, asking why they did not ensure that the company's insurance cover was sufficient to meet all eventualities. A spokesman for North West London strategic health authority said Brent medical ethics committee was responsible for assessing whether the company had appropriate insurance and had confirmed that TeGenero offered an indemnity in line with ABPI guidance. The company's liability to claims from the men was not limited to the sum covered by insurance, he added.