The firm behind a drug trial that caused six healthy men to suffer multiple organ failure was today cleared of giving them wrong doses or contaminated medicines.

The findings, by the Medicines and Healthcare Products Regulatory Agency - the government's drug watchdog - provoked claims of a whitewash.

It said the life-threatening inflammation of the men's tissues and internal organs were "most likely" caused by an unexpected biological effect that had not been detected in animal tests of the TGN1412 drug, manufactured by TeGenero.

"We are satisfied that the adverse incidents which occurred were not as a result of any errors made in the manufacture of TGN1412, its formulation, dilution or administration to trial participants," the agency's chief executive, Professor Kent Woods, said.

The conclusion was the same as that reached by the MHRA in an interim inquiry.

However, the watchdog criticised the research firm Parexel, which ran the trial, for failing to follow some procedures correctly and making errors over contracts and patient records.

Its report said the doctor who screened the volunteers before they began the trial did not have a contract or "adequate training and experience for the role".

The report also noted there was no "formal system" in place for 24-hour medical cover, and no contract in place between Parexel and TeGenero, although one was subsequently issued.

It said Parexel, based at Northwick Park hospital, in north-west London, had "failed to complete the full medical background of a trial subject in writing".

The two volunteers given a placebo instead of TGN1412 had been allowed to leave the premises "before appropriate checks" had been carried out to confirm they had not received the drug.

Some scientists have said the six volunteers should never have been simultaneously given the drug - which had never been tested on humans before - because of its unique nature.

It is one of 19 monoclonal antibody drugs currently in use in the UK, but the only one to stimulate rather than repress the immune system. It is designed to treat rheumatoid arthritis, leukaemia and multiple sclerosis.

Ann Alexander, of solicitors Irwin Mitchell, which is representing two of the victims, described the report as "totally inadequate" and a "whitewash".

"Today's report ... leaves very many questions unanswered," she said. "It gives no detailed information about the pre-clinical trials, about which there has been conflicting information since the trial was suspended.

"Were the animals involved appropriate for a comparison with human subjects? What were the effects on these animals?

"We question the report's findings that the reaction was unpredictable, and we call for independent scrutiny of these and many more questions raised but not answered by the report."

Ms Alexander said there was now evidence that the volunteers were not treated for up to five hours after first suffering they first suffered adverse symptoms.

Four of the men have been given unconditional interim payments of £10,000. One, 20-year-old Ryan Wilson, of Highbury, north London, remains in hospital.

"This is a complex scientific issue which raises important scientific and medical questions about the potential risks associated with this type of drug and how to make the transition from pre-clinical testing to trials in humans," the MHRA report said.

Following the organisation's interim report last month, the health secretary, Patricia Hewitt, set up an expert group on monoclonal antibodies treatment to investigate what happened and how to prevent it from happening again.

Its interim findings are due to be published within three months, the Department of Health said today.