The prime minister told the judge probing the death of weapons' expert David Kelly that he would have resigned had the government-dossier on Iraqi weapons of mass destruction been "sexed-up."
Making a historical court appearance under intense media glare Blair first had to face a gauntlet of hate from hundreds of anti-war campaigners before he walked inside the Royal Courts of Justice in central London.
Solemn-faced, he struck an indignant note, describing the accusation that the government had hyped the Iraqi threat as "an absolutely fundamental charge".
"This was an allegation that we had behaved in a way which … if true would have merited my resignation," he said.
Despite acknowledging that he was under intense pressure to make a strong case for disarming Iraq, Blair stood by the dossier which said Iraq could deploy weapons of mass destruction within just 45 minutes.
"We described the intelligence in a way that was perfectly justified," he said.
"The clamour for us to produce evidence was very strong," Blair said. "We had to disclose what we knew because there was an enormous clamour…it was important the dossier made the best case we could have," he added.
But Blair's government is far from silencing critics who opposed the war.
''We described the intelligence in a way that was perfectly justified''
UK Prime Minister
The criticism of the government has infact grown louder since Kelly, identified as the source behind a BBC report on "sexing-up" the dossier, was found dead with a slashed wrist.
The government came under fire for the way it handled the Kelly affair and Blair's own ratings have sharply declined as a result.
The discontent showed as Blair arrived at the Royal Courts of Justice in central London.
Hundreds of protesters jeered loudly, some chanting: "War criminal -Tony Blair." Some others carried anti-war placards, denouncing the prime minister.
Scores of other members of the public had queued overnight to guarantee a seat in the court, to witness the rare spectacle of a prime minister taking the stand before a judge.
The only other serving UK prime minister to appear before a judicial inquiry was John Major, who appeared in 1994 before a probe, concerning illegal arms sales to Iraq prior to 1991.