Tony Blair should call a full-scale inquiry to look at whether the UK went to war with Iraq on the basis of flawed evidence, Menzies Campbell has said.
The Liberal Democrat foreign affairs spokesman also used his conference speech to criticise the treatment of British citizens held by the US at Guantanamo Bay.
In other debates at the Brighton conference, delegates are expected to condemn a new extradition treaty with the US, call for compulsory sex education to begin aged seven and support a ban on smacking children.
Lib Dem leader Charles Kennedy, who will address the conference on Thursday, is also arguing that middle classes should be prepared to pay more to create a decent society.
Mr Campbell said: "We need a full-scale inquiry into the question of whether we went to war with Iraq on a flawed prospectus, either because of inadequate intelligence or the mishandling of intelligence once it had been obtained.
"I am also calling for the full text of the attorney general's opinion on the legality of going to war against Iraq to be published and in particular the factual basis on which he gave that opinion, including whether he did so on the basis of the dossier of the 24 September.
"Did he do so knowing the weapons were battlefield weapons and did not constitute a direct threat to the United Kingdom?
"Did he know that intelligence was based on hearsay and was single sourced?"
Mr Campbell - who is also Lib Dem deputy leader - said that the Hutton inquiry into the death of government weapons expert Dr David Kelly had made the decision to go to war a "live issue".
"We were very sceptical [about the case for war] and I think that scepticism has been entirely justified."
He said he believed the Liberal Democrat position on Iraq had played a part in boosting the party's popularity.
At the same time he believes the credibility of Prime Minister Tony Blair has been undermined - something that might affect voters in other areas.
"In the end it's the economy stupid - that's what Bill Clinton beat George Bush on ... but equally my own judgement is that foreign policy issues have an effect on credibility, have an effect on perception of the style and of the smell of the government."
Campbell wants the legality of the war scrutinized
Mr Campbell said that another problem for the government is that there is no apparent end in sight to involvement in Iraq.
But he warned that anyone tempted to suggest withdrawal should think again, as that would result in a political vacuum which would be occupied by former forces of Saddam Hussein's regime and Muslim extremists.
On Britain's damaged relationship with France and Germany in the wake of the Iraq war, Mr Campbell said it was now necessary to return to the UN route abandoned before the invasion of Iraq.
He called on Mr Blair to go in private to George W Bush and urge him to follow the instincts of Colin Powell, rather than the more hawkish Defence Secretary Donald Rumsfeld.
Mr Campbell also expressed outrage at the plight of the Guantanamo Bay prisoners, branding the situation "monstrous".
"If we were keeping American citizens on the Isle of Wight in similar conditions to which British citizens are being kept at Guantanamo Bay ... every senator and every congressman would be outraged, as would the White House."
Meanwhile, party leader Charles Kennedy said his party's plans to replace council tax with a local income tax would benefit those who needed it most, such as the elderly.
"Do the middle classes feel that we can afford as a society ... to carry on with declining public services over which people don't feel they have enough input and control and higher costs that have already been loaded on them ... of caring for elderly relatives, or financing their children through tertiary, university education?" he told BBC Radio 4's Today programme.