Four of the volunteers who were taken seriously ill during a first-in-man trial of an antibody drug - conducted by contract research organisation Parexel - have been given unconditional payments to help finance their recovery.

The £10,000 payment comes after a £5,000 offer by TeGenero, which manufactured the drug under test in the study, was turned down last month. The latest agreement also removes an earlier clause that would have prohibited the men from seeking compensation in the courts within the next year.

A total of six volunteers received the drug, TGN1412, in the study at a dedicated clinical unit run by Parexel at Northwick Park Hospital in North London. The suffered almost immediate adverse reactions, leading to multiple organ failure, which caused them to be hospitalised.

Five of the volunteers have since been released, but the sixth may have to stay in the hospital for as long as six months, and could lose the tips of three fingers, according to the hospital. Lawyers representing them have said they could be ill for life, and all are said to be seeking ‘substantial damages’.

The UK’s Medicines and Healthcare products Regulatory Agency (MHRA) is investigating the conduct of the trial, but in an interim report released last month said it seemed to be an unexpected side effect of the therapy, rather than a result of deviation from good clinical practice or a manufacturing problem.

The four men who have accepted the payments are all represented by lawfirm Leigh Day & Co. The remaining two – who suffered the worst side effects - have not accepted the deal, although their lawyers did not say whether they had been offered the same amount.

Martyn Day of Leigh Day & Co said the interim payment was made to allow the men to “finance the coming few didfficult months while we try and work out quite how bad is the cloud hanging over their heads following the drug trial."

TeGenero’s insurance company will cover the payments, but Day said the German biotechnology company had only taken out a £2 million cover for the trial via a no-fault arbitration scheme, which might not be enough to compensate the men for any possible long-term damage caused by TGN1412.

He also suggested the firm was looking into the possibility of filing claims against Parexel or the MHRA.