Facing interrogation at a potentially explosive inquiry on Tuesday, Alistair Campbell said he did not exaggerate the threat posed by Saddam Hussein to justify an unpopular war.
Tony Blair's media chief said he even urged intelligence chiefs to tone down colourful language from a dossier on Baghdad's weapons.
The prime minister used the dossier to justify his case for defying public opinion to join the US-led invasion of Iraq.
Campbell said he had "no input, output (or) influence" on the dossier at any stage, despite accusations by a BBC reporter that the hype was all his.
"I said: 'The drier the better, cut the rhetoric,'" Campbell said.
"There were areas where the language was too colourful. I also said the more intelligence-based it was, the better."
Arriving at the Royal Courts of Justice in central London, Campbell, 46, faced jeering from around 30 demonstrators waving banners protesting against the Iraq war.
But he repeated his denial that he gave undue prominence in the dossier to a claim that Saddam could launch weapons at 45 minutes' notice.
"There were areas where the language was too colourful. I also said the more intelligence-based it was, the better"
Tony Blair's press secretary
"I had no input, output, influence upon them (the words) whatsoever at any stage in the process," he said.
But Campbell acknowledged sceptical Britons may not be convinced.
Reading from his diary, he gave a flavour of how he feared trust was slipping away as the drama unfolded.
"It's grim," he said, quoting from his 30 May entry. "It's grim for me, It's grim for TB (Tony Blair)."
Campbell is at the centre of a vicious row with the BBC over a report that he "sexed up" the weapons intelligence to win over war sceptics.
Weapons scientist David Kelly, who was found dead with a slashed wrist near his home in July, was identified as the source for the report, just days before his death.
The inquiry into events leading to Kelly's death is a key test for Blair, who is due to testify after his holiday.