Alastair Campbell "momentarily" considered suicide during the Hutton inquiry, he reveals in his diary.
Mr Campbell was reportedly paid £1m for his diaries
Tony Blair's ex-spokesman described preparing to present his diaries to the inquiry into Dr David Kelly's death.
In his entry for 10 August 2003 he said he wondered "whether what I discovered on reading my own diary would be so awful that I would want to top myself".
Dr Kelly killed himself after being named as the source of a report into the government's Iraq dossier.
Mr Campbell wrote that he had left a holiday home in France on 10 August 2003 to pick up the diary to put forward as evidence for the inquiry.
'A passing thought'
"As I left the house, and said goodbye to Fiona (his partner), I did actually wonder momentarily whether it would be the last time I saw her, whether what I discovered on reading my own diary would be so awful that I would want to top myself.
"It was only a passing thought, but it was there, and it came back several times as I drove down to Marseilles.
"I knew I had done nothing wrong, but in this climate, things had gone beyond reason, it was like a drama or a novel, and nobody had control of events."
He also said Mr Blair had talked him out of resigning after Dr Kelly died, saying it would be a "disaster" for Mr Campbell.
Former BBC reporter Andrew Gilligan's report claimed a government dossier setting out details of Saddam Hussein's weapons of mass destruction (WMD) had been "sexed up" to try to convince the public to back military action against Iraq.
Mr Gilligan quoted an unnamed senior official involved in drawing it up as saying that parts of it - specifically a claim that Saddam could launch WMD at 45 minutes' notice - had been inserted against the wishes of the intelligence services.
The report led to a furious row between the BBC and Downing Street and the subsequent Hutton Inquiry in to the circumstances of Dr Kelly's death.
The inquiry ruled that Mr Gilligan's report had been wrong because Joint Intelligence Committee chairman John Scarlett had had ownership of the dossier and had agreed to everything included in it.
The diary covers events from 1994, when Mr Campbell was recruited as Mr Blair's press secretary, to 2003, when he resigned as the PM's spokesman.
Among other disclosures are an admission that he "screwed up" by leaking a story to the Times that the government was effectively ruling out UK entry into the Euro in Tony Blair's first term as prime minister.
Mr Campbell wrote that he and Charlie Whelan, Gordon Brown's then adviser, thought - wrongly - that they had their bosses' approval.
"I suddenly realised that because I had not really checked and double-checked with TB, we had briefed an enormous story on the basis of a cock-up," Mr Campbell wrote.
A few days later Mr Brown confirmed, in a statement in the House of Commons, that Britain would not be among the first wave of countries joining the euro.
Among other disclosures it says civil servants had planned for a caretaker government led by John Prescott in case Mr Blair quit after losing a vote on the Iraq war.
In 2003 Cabinet Secretary Andrew Turnbull was "quietly looking into" the scheme, Mr Campbell says.
Mr Prescott could have replaced Mr Blair, Alastair Campbell says
Mr Blair eventually won the Commons vote on the war, but with 139 Labour MPs rebelling against the government.
The book also describes the Cabinet meeting the day before the MPs' vote in 2003 and following Commons Leader Robin Cook's resignation over Iraq.
Then Labour Party chairman John Reid, who stepped down at the same time as Mr Blair, warned ministers "we will be judged by the Iraq that replaces Saddam's Iraq, and by the Middle East".
'No going back'
Clare Short told the meeting that she was "going to have my little agonising overnight. I owe it to you".
Mr Campbell writes: "All of us, I think, had had pretty severe moments of doubt but he [Mr Blair] hadn't really, or if he had he had hidden them even from us.
"Now there was no going back at all."
Mr Campbell also reveals that in 2002 Mr Blair was considering resigning before the 2005 general election.
The diary was published on Monday, weeks after Mr Blair stepped down as prime minister, and Mr Brown took over.
Mr Campbell has admitted removing references to Gordon Brown to stop Tory leader David Cameron thinking he had "a gold-mine to use against the new Labour prime minister".
The book leaves out much detail of the relationship between Mr Brown and Mr Blair. Mr Campbell told BBC Radio 4's Today programme these diaries were "the first chapter of a record that I intend to put into the public domain".
Asked about rows with journalists, he said: "I've never really worked out why I provoked such strong reactions."
He also played down the significance of any changes to the workings of the Downing Street communications unit made by Mr Brown since becoming prime minister.