A Canadian military withdrawal from southern Afghanistan could trigger a domino-effect collapse of Nato alliance support for the mission, Ottawa's Defence Minister has warned.

Peter MacKay told MPs that history would judge them harshly if they opted to cut the Canadian commitment short because of mounting casualties and set the scene for the abandonment of Western efforts to rebuild the shattered Afghan economy and infrastructure.

With 2500 troops in Kandahar, the heartland of the Taliban, and 78 dead so far, Canada's minority government is facing increasing pressure to pull out unless other Nato allies such as Germany or France commit combat forces to ease the burden of the counter-insurgency campaign.

Mr MacKay said: "This is perhaps the most important debate facing our parliament and our nation today. The consequences of pulling Canada's military out of Afghanistan could have a far-reaching effect or a domino effect on others.

"If we were to pack up and leave Afghanistan, why wouldn't other nations follow suit? How would history judge us if Canada walked away from Afghanistan?"

The main opposition Liberals agreed with ruling Conservatives on the need to maintain troops in Afghanistan to 2011 only if Nato allies send reinforcements soon.

Now they have opened a new debate on whether Canadian soldiers should continue hunting insurgents beyond their current mandate of February 2009, or reduce the mission to a non-combat training role in Kandahar province with a cut-off date of 2011.

Sir Menzies Campbell, the former LibDem leader, said: "Bob Gates, the US Defence Secretary, has already issued an unequivocal wake-up call to the members of Nato.

"There is a real risk of a two-tier alliance divided between those who are willing to fight and those who are not.

"Even British public support for our commitment in Afghanistan could easily become eroded if the UK is seen to be assuming disproportionate responsibilities. Nato cannot afford to fail in Afghanistan, but the sharing of the burden has got to be on an equitable basis."

The UK has 7100 soldiers in Helmand and has lost 89 since the US-led invasion in 2001. Almost all of those killed in action have died since 2006.