The Australian Government's case for war in Iraq has been "blown apart" by a report finding no link between Iraq and al-Qaeda, the Labor opposition said.
Mr Howard was a strong advocate for war with Iraq
A US inquiry into the September 11 attacks found "no credible evidence" that Saddam Hussein helped al-Qaeda.
Labor spokesman Kevin Rudd said the findings destroyed the government's claim that the war was part of a wider campaign against global terrorism.
But the government insisted it had never linked Iraq with the US attacks.
Prime Minister John Howard's decision to join the US-led war on Iraq has divided Australia.
The Labor Party has repeatedly criticised the move, and earlier this month Labor leader Mark Latham reaffirmed his commitment to withdraw Australian troops if he won the general election due later this year.
The US commission report concluded that al-Qaeda's leader, Osama Bin Laden, explored the possibility of co-operation with Iraq when he was living in Sudan in the early 1990s.
Further discussions are said to have taken place when he returned to Afghanistan, but the report concluded that none of these contacts "appear to have resulted in a collaborative relationship".
Labor's foreign affairs spokesman Kevin Rudd said the report showed that government claims linking the Iraq war with a wider campaign against terrorism had been proved false.
"It's quite plain from what's been produced in the United States that the core argument advanced by John Howard - that attacking Iraq was part of the war against terrorism - has been blown apart by this report," he said.
But Defence Minister Robert Hill insisted that Australia went into the Iraq war because of the United Nations' inability to do anything about its alleged weapons of mass destruction rather than any links to terrorist groups.
In the run-up to the US-led invasion, Mr Howard did loosely link al-Qaeda to Iraq.
In an address to the nation in March 2003 he said: "If we don't make sure that Iraq is disarmed, that of itself will encourage other rogue states to acquire and develop weapons of mass destruction".
"When you have on top of that clear evidence... that al-Qaeda, the most lethal of the international terrorist organisations, wants to get its hands on, and in fact is doing its own work in relation to those weapons, that to me is pretty compelling," he said.
But Mr Howard is never thought to have specifically linked the events of September 11 to the Iraqi regime.