ESQUIMALT, B.C. — Prime Minister Stephen Harper says he has it in writing that Canada will be exempted from the staggering development cost increases associated with the F-35 stealth fighter.

He lamented Friday that the ultra-high-tech jets and their enormous price tag had become a political football in the race toward the May 2 election.

"You have to understand that in terms of the F-35 costs, we've been very detailed with those to the Canadian public," Harper said after releasing the Conservative platform in Mississauga, Ont.

"A lot of the developmental costs you're reading in the United States, the contract we've signed shelters us from any increase in those kinds of costs. We're very confident of our cost estimates and we have built in some latitude, some contingency in any case. So we are very confident we are within those measures."

Canada signed a memorandum of understanding with Washington last summer that signalled its intention to buy 65 of the F-35 Lightnings, but it has yet to sign a contract for the actual sale.

Alan Williams, a former senior official and vocal critic of the program, said Harper is correct that allies are "sheltered" from development costs, but the federal government has no such guarantee over rising "procurement costs."

U.S. export law forbids the Pentagon from selling military equipment at a cost less than it pays.

The most recent report by the U.S. Government Accountability Office, issued Thursday, red-flagged concerns that the F-35 prime contractor did not have a system in place to "effectively track costs and control schedules."

The 52-page analysis also expressed concern at the estimated 10,000 design changes Lockheed Martin was expected to impose between now and 2016, when the aircraft is supposed to exit the development phase.

The respected U.S. budgetary watchdog warned that the F-35 may not live up to its billing.

Despite that, Harper insisted the air force needs the planes.

"I think it's in a way sad that the new jets have become an issue in this campaign," he said. "We've got our men and women in uniform up there flying airplanes that we know at the end of this decade are going to have to be replaced, and the thing we owe to them is to replace those airplanes."

The question of whether the country needs the stealth fighters was at the centre of an NDP defence policy released Friday in Esquimalt.

New Democrat Leader Jack Layton, like the Liberals, promised to halt the purchase and hold an open competition to replace the existing CF-18s, which are expected to reach the end of their service life between 2017 and 2020.

But Layton qualified his position within the framework of a full defence policy review, a white paper which would lay out the country's military needs.

"We have to got have a full discussion across Canada about what our defence priorities are," he said at an event overlooking Canadian Forces Base Esquimalt.

"It's time to go back to Square 1. We believe there are other priorities for investment in military procurement, particularly ships that are so vital in dealing with peacekeeping missions and (humanitarian) disasters around the world."

The Harper government has been under almost continuous assault since it signalled last year it intended to buy 65 of the F-35s, starting in 2016-17. The estimated cost -- $9 billion to purchase and $7 billion for 20 years of maintenance -- has been the subject of furious debate and contradiction.

The most dramatic has involved the parliamentary budget officer, who estimated before the election call that the total bill for the jets over 30 years could reach $29.4 billion.