Attorney General Dominic Grieve has ruled out asking the High Court to order an inquest into the death of David Kelly.
David Cameron suggested a full inquest was unnecessary last month, saying the Hutton report into the Government weapons inspector's death had been "fairly clear".
But a group of campaigning doctors, led by Dr Stephen Frost, accused the Government of being "complicit in a determined and concerted cover-up", saying they will now seek a judicial review of Mr Grieve's decision.
Speaking in the Commons, Mr Grieve told MPs the evidence that Dr Kelly took his own life was "overwhelming", adding that there was no evidence to support claims he was murdered or "any kind of conspiracy theory".
The scientist's body was found in woods close to his Oxfordshire home in 2003, shortly after he had been revealed as the source of a BBC report questioning the accuracy of a government dossier arguing the case for war in Iraq.
The Hutton Inquiry in 2004 found that Dr Kelly committed suicide, and then-justice secretary Lord Falconer ruled the inquiry could take the place of an inquest in the coroner's court. But the doctors pointed out that Lord Hutton spent only half a day of his 24-day inquiry considering the cause of Dr Kelly's death.
They have denounced the Hutton report as a "whitewash" which "failed adequately to address the cause of death itself and the manner of death".
They argued: "No coroner in the land would have reached a suicide verdict on the evidence which Lord Hutton heard. The coroner is required to hear evidence which constitutes proof beyond reasonable doubt that the deceased killed himself and that he intended to kill himself, before he may return a verdict of suicide. Lord Hutton did not hear evidence which came near to satisfying that test."
Dr Frost said: "It is therefore very surprising and perplexing that the Attorney General today supports those who wish to deny Dr Kelly a proper inquest. This is clearly a political decision when it should have been a decision based solely on the law."
He condemned the "deeply flawed" decision, saying it had "no basis in law", and called for Mr Grieve to resign.