Dr Kelly, a Ministry of Defence biological warfare expert who questioned the government's justification for invading Iraq, died soon after he was exposed as the source for a critical BBC report on the war in 2003.

The controversy surrounding his death only deepened after an official inquiry, led by the then prime minister Tony Blair's close friend Lord Hutton - and immediately branded a "whitewash" by anti-war campaigners - claimed that the scientist had taken his own life by cutting his wrist after overdosing on painkillers.

But a group of 13 sceptical doctors - led by retired orthopaedic and trauma surgery consultant David Halpin - have now mounted a legal challenge to overturn Lord Hutton's claim that Mr Kelly committed suicide.

Pointing out that, unusually, no coroner's inquest had been held into the scientist's death, Mr Halpin explained that "Lord Hutton's inquiry did not have the same legal standards as a coroner's inquest."

"As a result, due process has been subverted, and the group of doctors that I am part of is not prepared to let that go," he stressed.

In 2003, after taking evidence from the pathologist who carried out the post-mortem examination into Dr Kelly's death, Lord Hutton claimed that "the principal cause of death was bleeding from incised wounds to the left wrist," combined with taking a large number of painkillers.

But Mr Halpin dismissed that explanation as inadequate and detailed how the group of 13 specialist doctors had compiled a medical dossier that rejects such a conclusion.

"Such a cut to the ulnar artery, which is small and difficult to access, could not have caused death. The bleeding from Dr Kelly's wrist is unlikely to have been so voluminous and rapid that it was the cause of death," he insisted.

"There is evidence of a cover-up and I think it is highly likely that Dr Kelly was assassinated," Mr Halpin asserted unequivocally.

Dr Kelly had been seconded to the United Nations from the Ministry of Defence to work as a weapons inspector in Iraq in 2002 and 2003, during the months that Tony Blair and then US president George W Bush were plotting to invade the country.

Alarmed that the British government was deliberately exaggerating the military threat from Iraq, Mr Kelly confided to a BBC reporter that ministers "probably knew" that their notorious claim that Iraq could attack Britain with missiles within 45 minutes was a lie.

Soon after his comments were broadcast and Mr Kelly's MoD bosses demanded to know more about his contacts with journalists, the scientist was found dead in a field near his home.

"Dr Kelly was a skilled and courageous man and he deserves a proper inquest," Mr Halpin declared.

He was joined by Liberal Democrat MP Norman Baker, who demanded that the government not only investigates the weapons inspector's death but also set up an inquiry into why Britain went to war in the first place.

"All we are asking for is proper legal process because we have not had it yet."