No-one could have predicted Dr David Kelly might commit suicide, Lord Hutton has said in his report.
Dr Kelly was said to have felt under immense pressure
The judge said no-one else was involved in the death of the government scientist who was found dead in woods near his home.
Lord Hutton said he believed Dr Kelly had killed himself after being named as the suspected source of the BBC's controversial weapons dossier story.
The law lord said he agreed with a psychiatrist that Dr Kelly was not suffering from a mental illness.
Suicide expert Professor Keith Hawton said Dr Kelly may have been affected by a fear of public "disgrace".
Professor Hawton had also said the pressure of the intense media coverage - after Dr Kelly was named as the source for BBC reporter Andrew Gilligan's story about the Iraq dossier - may have contributed to his state of mind.
Lord Hutton said Dr Kelly had been a devoted husband and father and a public servant who had served with "great distinction" and that his death was a "great tragedy".
He said: "Whatever pressures and strains Dr Kelly was subjected to by the decisions and actions taken in the weeks before his death, I am satisfied that no one realised or should have realised that those pressures and strains might drive him to take his own life or contribute to his decision to do so."
Minstry of Defence officials had been concerned about Dr Kelly's mental wellbeing, said the judge.
But he argued: "Because of his intensely private nature, Dr Kelly was not an easy man to help or to whom to give advice."
The judge cleared the government of "dishonourable or underhand or duplicitous" behaviour in the naming of Dr Kelly.
He said the government had been concerned it would be "charged with a serious cover-up" if it did not reveal that a civil servant had come forward, given the "grave" nature of the allegations made against the integrity of the government.
It also feared Dr Kelly's name would be disclosed by the media.
But he did say the MoD was at fault in the way it dealt with Dr Kelly once his name was made public and in failing to tell Dr Kelly his name would be made public.
Referring to Dr Kelly's fateful meeting with Mr Gilligan, Lord Hutton indicated the weapons inspector may have realised he had said too much.
"Dr Kelly's meeting with Mr Gilligan was unauthorised and in meeting Mr Gilligan and discussing intelligence matters with him, Dr Kelly was acting in breach of the Civil Service code of procedure which applied to him...
"It may be that when he met Mr Gilligan, Dr Kelly said more to him than he had intended to say and that at the time of the meeting he did not realise the gravity of the situation which he was helping to create by discussing intelligence matters with Mr Gilligan."