Senior defence officials concede there is currently no Plan B -- or back-up proposal -- to deal with delays in the F-35 jetfighter program, and insist one is not necessary because of recent upgrades to the CF-18 fleet.

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The U.S. Air Force released this July 2011 photo of its new F-35 Lightning II joint strike fighter (JSF) at Eglin Air Force Base, Fla. Detractors say the F-35 stealth fighter, the costliest military plane ever, is destined to go down as one of the biggest follies in aviation history. (AP Photo/U.S. Air Force, Samuel King Jr., File)

But there were suggestions Wednesday the Harper government might be casting around for an alternative as Julian Fantino, the associate minister of defence, told opposition parties to "stay tuned" for a response to the Pentagon's statement that the cost of the radar-evading plane would rise.

The Conservatives have been hammered for months in the House of Commons over delays and cost overruns in the multinational project.

Other allies, such as Australia, have placed orders for Super Hornets -- the newer, beefed up version of the CF-18 -- to hedge against F-35 delays.

It's not necessary for Canada to go down that road, said a high-level defence official, who spoke on condition of anonymity.

"We have no back-up plan. We have a reserve and flexibility in the life extensions we've done structurally to our F-18s and in weapons systems," said the source, referring to the $1.8-billion modernization that's took place over the last decade.

The upgrades mean the current fighter-bombers can "easily" stay in the air to 2020 and beyond.

The Royal Canadian Air Force isn't expected to take delivery of its first F-35 from U.S. defence giant Lockheed Martin until 2016, with the bulk of the planned 65 planes arriving around 2020.

"The (CF-18) will still be effective, but you'll have slightly lower numbers as we get to maximum fatigue life on air frames," said the source in a recent background interview.

Canadian order 'evolving'

Whether the military can do with fewer F-35s has been a matter of intense debate for months. Internal air force documents, released last fall, showed planners consider 65 to be the "minimum acceptable fleet" and going below presents risks and could even jeopardize the ability to carry out a full-range of missions.

Neither Fantino, nor Defence Minister Peter MacKay, would say whether a smaller order is in offing because of increased costs.

But it's clear the Conservatives are feeling heat on the issue with caucus members expressing private dismay at how the U.S. program seems to have spiralled out of control and fallen off schedule with a spectacular stream of development glitches.

"It's very simple," said interim Liberal Leader Bob Rae, who fired repeated questions at Fantino, hoping to knock him off his speaking notes.

"We've been raising this as party for 18 months, trying to get a clear answer. We don't need the robo-call answer. We need a real answer to these questions. How many planes? At what price? And when are they going to be delivered?"

New Democrat MP Matthew Kellway was even more blunt: "Will the minister give Canadians a straight answer? Are they ordering the same number of planes? Yes or no?"

It's been suggested in political circles that unmanned aircraft might be able to make up the difference, but those in uniform dismiss the idea.

"We don't have this notion of a Plan B. Currently Canada doesn't see the need for it (because) we are committed to the F-35 program," Lt.-Gen. Andre Deschamps, head of the Royal Canadian Air Force, said in a recent interview with The Canadian Press.

"I think there's a lot of confusion around what fighters can do.

The foremost job of any fighter aircraft ... is air control, which is fundamental to any sovereign action; maintaining control of your own air space.

"Right now, the only tool that's fully effective right across the spectrum of air control is a fighter -- a manned fighter."

The air force has been conducting a study since last spring on what the right balance might be between manned and unmanned aircraft.

"UAVs are good at doing the dull, dirty and dangerous stuff," Deschamps said. "UAVs are optimized for that mission."

Unmanned aircraft, which Canada plans to purchase some time down the road, would be armed, but only for air-to-ground attacks and the technology allowing for air-to-air interception does not exist, said the air chief.

"They can't fulfil any of the fighter functions right now," said Deschamps.

Canada is expected to chair a meeting in Washington some time in the next few weeks among F-35 partner nations to talk about the latest developments in the program. A wider, more detailed conference is planned for Australia next month.