A leaked evaluation in Washington criticizes the visibility in the cockpit of the multi-role fighter, and contains blunt comments from test pilots that suggest the shortcomings could get planes shot down in combat.
The design prevents pilots from looking behind them.
"The head rest is too large and will impede aft (rear) visibility and survivability during surface and air engagements," one test pilot was quoted as saying in the U.S. Defense Department's directorate of operational test and evaluation report.
A second pilot reportedly said visibility is crucial and any disruption "will get the pilot gunned" down in dogfights.
The document was leaked and posted online Wednesday.
Pilots have also cited concerns about the sophisticated helmet that's supposed to display data, saying there are flickering and non-existent readings.
Vocal critics, such as aviation expert Winslow Wheeler, say the F-35 is not ready for combat training, let alone combat.
Both the New Democrat and Liberal defence critics say the stealth fighter should be dropped from consideration.
A spokeswoman for Public Works Minister Rona Ambrose accused the Opposition of playing politics.
Michelle Bakos said it's "shameful that the opposition is trying to undermine the National Fighter Procurement Secretariat."
The government is committed to a thorough review, she said.
"The plan will ensure that the Royal Canadian Air Force acquires the fighter aircraft it needs to complete the missions asked of it," said Bakos "The Harper Government is committed to examining all options and has made significant progress."
The Harper government has asked a panel of independent experts to ovesee an evaluation of all the options on the market to replace the 1980s vintage CF-18 fighters to ensure the process is rigourous and fair.
Once the evaluation is complete, the government will decide whether it will hold a full-fledged competition.
The Public Works secretariat overseeing the fighter replacement recently sent a questionnaire to the five companies that could be bidding, asking for detailed information and plans, including costing and availability.
Last year's auditor general report, which accused National Defence of hiding the full cost and Public Works of not following the proper process, forced a reset of the program.
The Lockheed Martin-built F-35, with a history of development delays and cost overruns, is still considered a strong contender. The Harper government initially chose it in 2010, and has expended much political capital defending the decision.
A spokesman for Lockheed Martin said the stealth jet has been declared ready for training, and that the company is "maturing" its operations and maintenance procedures.
Peter Simmons defended the program.
"The (U.S.)Air Force concluded through its Operational Utility Evaluation that the F-35 is ready to conduct safe and effective flying training operations," said Simmons.
NDP procurement critic Matthew Kellway, however, said Lockheed Martin has a brochure while the other competitors have real planes.
"I think the challenge for the government now is: How do you run a legitimate competition that pits a paper fighter against (other) real, operational fighter jets that can be flown and tested?" he said.
One of the options, long considered, is extending the life of the CF-18s beyond 2020 until the stealth fighter is fully tested and ready for operations.
Engineers would have to find a way to extend the CF-18 airframe life, technically complex and expensive.
New Democrat defence critic Jack Harris says that given the latest report by the Pentagon, an extension is unrealistic.
"There are planes that are already available and flying," he said.
"The government needs to have a good, hard look at what they've committed themselves and say this plane is not going to be available to us when we need it."
Note to readers: This is a corrected story. An earlier version incorrectly said a panel of experts would be producing an evaluation report. An earlier version also attributed NDP procurement critic Matthew Kellway's comments to Liberal MP John McKay.
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