Charles Kennedy told the Lib Dems' Brighton conference that Britain would not have gone to war with Iraq if Parliament had "known what the country now knows".
The Lib Dem leader recalled the moment he was handed the "sexed up" September dossier in a Brighton hotel room by a special branch officer exactly a year ago.
The dossier contained the widely-criticised claim that Saddam Hussein could fire weapons of mass destruction within 45 minutes.
Mr Kennedy, who had to leave last year's conference to attend an emergency recall of the House of Commons to debate the dossier, suggested MPs would have voted differently with the benefit of hindsight.
Mr Blair only narrowly avoided defeat in a Commons vote opposing military action by 296 votes to 217.
Mr Kennedy said: "If Parliament had known what the country now knows, the very vote which led us into this unnecessary conflict could and should have had a very different outcome."
Liberal Democrat Agenda for today:
Council Tax, 9am: Plans to replace council tax with a local income tax of up to 3p in the pound will be the centrepiece of the next Lib Dem election manifesto.
The economy, 10.45am: Treasury spokesman Matthew Taylor will attack Gordon Brown's record followed by a discussion about the euro.
University fees, 2pm: Discussion of plans for a more flexible higher education system with more part-time study and vocational training as well as criticism of Labour's student tuition and top-up fees.
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