The former head of Canada's military says the Liberal government’s plan to buy an interim fighter jet fleet will be expensive and difficult to carry out.

Retired general Tom Lawson, who served as chief of the defence staff from 2012 to 2015, says he believes Canada will end up buying Lockheed Martin's F-35 fighter jet to replace the current fleet of CF-18 Hornets despite the government choosing Boeing's Super Hornet to beef up the current air craft.

"There are hundreds of these things flying now and they've been declared combat-capable in the United States," Lawson said in an interview with Evan Solomon, host of CTV's Question Period.

"And they'll be getting better and better for years to come. The F-35 is the way forward, not only for the United States but all the members and the partners who have bought into that program."

Lawson, a long-time supporter of the F-35, is now retired from the Canadian Forces. In an email to CTVNews.ca, he said he has done several days of work for Lockheed Martin as a strategic adviser, but noted his support for the F-35 dates back to his time as assistant chief of the Royal Canadian Air Force.

The F-35 was the source of considerable controversy during Lawson's time as Canada's top soldier, after the auditor general and parliamentary budget officer found a range of problems with a planned purchase of the Lockheed Martin jets. The problems included national defence not telling the government about the aircraft's potential drawbacks or the full costs of the program, as well as not having documents to support some of the decisions defence officials made.

Prime Minister Justin Trudeau promised, during the 2015 election campaign, to hold an open competition to choose the CF-18 replacement, but pledged at the same time not to buy the F-35.

Last November, the government announced it was working on buying 18 Boeing Super Hornets as an interim measure while holding a five-year competition to choose a long-term replacement for the CF-18s.

Lawson, a former pilot, said that decision caught him off-guard because he believes Canada will eventually purchase the F-35.

"Every time the F-35 is in competition, it is the top -- in many cases the only choice out there, and it's becoming cheaper and developing an excellent reputation," Lawson said.

"So I think what caught me by surprise is that this putting off of that decision will of course be expensive and very difficult for the RCAF to carry out."

The current fleet of CF-18s is more than 30 years old and down from 138 planes to 76, according to numbers provided by the government.

During the 2015 election campaign, the Liberals estimated the cost of an F-35 at $175 million per plane, with the Super Hornet coming in at $65 million.

Canada has obligations to NORAD and NATO that require a specific number of jets to be "mission-ready," although officials won't say how many planes that means. Defence officials say there aren't enough mission-ready planes, but admit that is due to a change in policy under which they started counting the combined NORAD and NATO commitments. That policy changed on the day they announced the plan to purchase an interim fleet, a defence spokeswoman told CTVNews.ca.