Ottawa has rejected any Canadian participation in a U.S.-led assault on Iraq.
Prime Minister Jean Chrtien made the announcement on Monday, as the House of Commons resumed sitting after a two-week break.
Chrtien said without the backing of the United Nations, Canada can't go along with any war initiative.
Critics have accused Chrtien of wavering and waffling on his position. Now there is no doubt where Canada stands.
"If military action proceeds without a new resolution of the Security Council, Canada will not participate," the prime minister said.
- Key Documents: Full text of Jean Chrtien's statement
This was greeted by loud, sustained applause from Liberal backbenchers. Members of the NDP and the Bloc Qubcois joined in, but not the Conservatives or the Canadian Alliance.
The government position remains weak, said Alliance leader Stephen Harper.
"I can tell you that in this political party we will be cheering for the success of our allies," Harper said. "If the Liberals are genuinely neutral, or will be cheering for Saddam Hussein, they should have the guts to say so. My guess is they don't."
Stockwell Day, the foreign affairs critic for the Alliance, said Canada is now on the side of some questionable regimes.
"The prime minister has thrown Canada onto the side of nations like Libya, Syria, China, nations who don't want to see a united front against Saddam Hussein," Day said.
The leader of the NDP, Jack Layton, said the Liberals should join France and Germany in speaking out against the war.
There still are questions about whether Ottawa is supporting the war through the back door. There are 31 officers with the U.S. troops as part of an exchange program, and a handful of Canadian frigates are deployed in the region.
Defence Minister John McCallum said the exchange officers are involved in logistical and support functions, and the ships are needed to fight the wider war against terrorism.
"The terrorist risk will, if anything, be greater than before as a consequence of war," McCallum said. "So for us to cut and run when the terrorism risk is greater would not be compatible with Canadian traditions."
Chrtien said he informed U.S. President George W. Bush a year ago that Canada wouldn't go to war without a UN resolution.
With the final collapse of diplomatic moves at the UN he said he's just making it official.
While the prime minister was distancing Canada from any involvement in a war, Australia and Poland announced they would commit troops to the fight.