OTTAWA — Federal Liberals plan to open a second front in their document war with the Harper government.

The party's defence critic, Dominic Leblanc, is demanding the release of a key air force report that lays out the justification for the purchase of F-35 stealth fighter jets.

The statement of operational requirements was stamped classified by National Defence last year and the Conservative government has resisted calls by the opposition parties to make the document public.

LeBlanc served notice to the House of Commons defence committee that he'll table a motion demanding the release of the statement -- a measure he hopes the NDP and Bloc Quebecois will support.

It's unclear when the committee will consider the motion, but it sets the stage for another tug-of-war over document secrecy.

Last year, Liberal MP Scott Brison brought forward a motion at the Commons finance committee demanding the Conservative government reveal projections on what its tough-on-crime legislation and corporate tax cuts will cost.

He won enough support to bring the matter before the Speaker of the Commons, who will hear arguments next week on whether the government should be held in contempt.

Defence Minister Peter MacKay justified keeping the F-35 fighter jet requirements secret last fall by saying that the document contained copyright material.

A member of the air force's F-35 planning team also suggested in testimony before a Commons committee that the stealth technology meant the requirements are highly classified.

The Defence Department has said it does not make operational requirement documents public, but it has released declassified versions for projects such as the 2006 sole-source purchase of Chinook helicopters.

The Liberals have said they would cancel the planned F-35 purchase and hold an open competition to replace the air force's aging CF-18 fleet.

A spokesman for MacKay said that would cost taxpayers $1 billion in lost royalties, tied to Canadian research and development on the aircraft, as well as drive up the purchase cost of the replacement jet.

"Michael Ignatieff's Liberals want to cancel the F-35 procurement to run another competition -- a competition that will inevitably result in the procurement of the F-35," said Jay Paxton, MacKay's director of communication.

"Except, the second time around Canada will need to buy the F-35 through foreign military sales."

But LeBlanc described the stealth fighter as a flying "credit card," saying the federal government has no idea how much it will eventually cost taxpayers.

Paxton denied that and said the public has already seen what it will cost.

Defence officials say the $9-billion price tag is firm, even though the cost per aircraft has yet to be made final. They also estimate the advanced fighter will require an additional $5 billion in maintenance over 20 years.

But LeBlanc said those are only "guesstimates."