OTTAWA — The Conservative government scrambled Tuesday to explain a report that the Afghanistan mission will run $1 billion over budget this fiscal year.

The government did not deny the budget blowout for 2007-08 reported in Montreal's La Presse newspaper. It simply warned that the $1 billion was based on preliminary estimates that cannot be confirmed until after the end of the fiscal year later this month.

The report came two days before a scheduled confidence vote in the House of Commons on extending Canada's military mission in Afghanistan.

It also came as John Manley, who headed a panel on the future of the mission, said violence will drag on in the wartorn country unless there's a political agreement with the Taliban.

Documents obtained under the Access to Information Act indicate the mission has cost Canadian taxpayers at least $7.5 billion since 2001 - double what was budgeted.

The documents say the mission cost $538 million more than expected over the first six months of the current fiscal year, and is projected to overshoot its budget by another $539 million by March 31.

"I wouldn't say that that's necessarily accurate," Defence Minister Peter MacKay said of the 2007-08 projection.

"That's based on a number of assessments that really are speculating right now on what the final costs are going to be over a full year."

The Defence Department says the projected budget for the Afghanistan mission through 2009 was $4.5 billion, but was adjusted to $5 billion because of the additional equipment purchased to protect soldiers. It says the increase does not count as a cost overrun.

The $5 billion figure, however, is at adds with the $6.3 billion cited by former defence minister Gordon O'Connor in a post-budget hearing last spring before the House of Commons.

Government sources say cost overruns have been an issue. Senior government officials say the military was warned last fall to keep its spending under-budget for 2008-09 after exceeding projections in previous years.

The NDP raised the latest projected cost over-runs in its latest attack on the efficiency of the mission.

New Democrats and the Bloc Quebecois are dead-set against a mission extension and are poised to vote against it Thursday.

The Conservatives are staunchly in favour and have managed to reach a deal with the Liberals, who have lately been backing down in any confidence vote that could trigger an election.

Thursday's vote would extend the military mission in Kandahar province until 2011.

The Tory-Liberal co-operation has given the NDP a hot-button issue with which to attack both of its rivals - and the cost overruns conveniently provided them with political ammunition just before the vote.

"These are numbers that of course the government wasn't telling us about, wasn't telling Canadians about," NDP Leader Jack Layton said.

"It's the mismanagement of the war. The costs are spiralling out of control just as we've seen by the Americans in Iraq and the government didn't deny this."

Members of the blue-ribbon Manley panel on Afghanistan appeared at a parliamentary committee Tuesday to discuss their findings. They stressed the non-military aspects of the mission.

Pamela Wallin spoke of her conversations with soldiers who described how moved they were to see classrooms full of Afghan girls.

Manley said that while some members of the hardline former Taliban regime deserve to be hauled before the International Criminal Court for human-rights atrocities, peace can only be achieved with some Taliban co-operation.

"This insurgency - unless it's the first one ever - will not end via military success," Manley said.

"It will end because of a political agreement that will resolve some of the issues that are there. . .

"We mustn't get ourselves into the position . . . where we're prepared to fight to the last Taliban, because quite frankly, we will never reach that point."