Going to war with Iraq was the "most difficult decision" of Tony Blair's life, his former media chief Alastair Campbell has told the BBC's Sunday AM.
But the decision was "driven by the right motives", Mr Campbell said.
Speaking ahead of the publication of his diaries, he insisted the Hutton Inquiry had been right not to blame Mr Blair for the death of Dr David Kelly.
But aside from a few exceptions, the weapons expert's suicide was "the worst period of my life", Mr Campbell said.
"I was a player in a series of events that somehow or other led to a man deciding he should kill himself," he added.
Dr Kelly killed himself after he was exposed as the source of allegations made by the BBC that intelligence justifying the war in Iraq had been knowingly "sexed up".
Mr Campbell insisted he had acted properly, but added: "All of us have to accept that as that was happening there was stuff going on that was leading that particular individual to feel despair."
In 2002, Mr Campbell said Mr Blair was considering resigning before the 2005 general election.
"I think it was the one period in his premiership when he really felt the pressures of the job. He was - for him - quite down about things.
"I think he was thinking... about whether it would give him more ability to push through domestic reform."
Mr Campbell said the former prime minister thought going to war in Iraq was the "right thing to do".
"I was alongside Tony as he made what was clearly the most difficult decision of his life and of his career.
"I hope (my book) gives people a sense of just how seriously he took that decision."
'Out of business'
The diaries, The Blair Years, also recall the former prime minister's idea to recruit then Liberal Democrat leader Paddy Ashdown in 1997.
Mr Campbell writes: "He stunned me straight out with the boldest plan yet. 'How would people feel if I gave Paddy a place in the Cabinet and started merger talks?'
"He was making a cup of tea and chuckling, 'We could put the Tories out of business for a generation.' "
Mr Campbell defended his decision to publish his diaries and denied he been asked to remove references to Gordon Brown.
But he had done so willingly, he said, to stop Tory leader David Cameron thinking he had "a gold mine to use against the new Labour prime minister".