OTTAWA -- Defence Minister Harjit Sajjan says the navy's submarines are "critical" for Canada's defence, but that the government has not decided whether to spend more money to keep them for the long term.

"Our submarines do play a critical role for sovereignty, but no decision has been made at this time," Sajjan said from London, where he was attending a peacekeeping conference. "As we complete the defence policy review, we'll have recommendations in there on the route we will be proceeding."

Documents obtained by the Canadian Press show that the first submarine will reach the end of its service life in 2022, with the other three retiring one by one until 2027.

The navy estimates that extending their usefulness would cost between $1.5 billion and $3 billion, depending on the upgrades that are made and how long they are to remain in service.

The Liberal government is currently developing a new defence policy, which will spell out what jobs the military will be expected to perform. That will have direct bearing on the types of equipment purchased in the coming years.

But any investment is likely to stoke controversy as the submarines have been plagued by technical problems since they were bought used from the United Kingdom in 1998 for what the Chretien government described as a bargain $750 million.

While naval officials say they have managed to fix many of the problems and have started using the ships in earnest, two were docked early this year over concerns about shoddy welding. Another had to be repaired after breaking down en route to a training exercise in Norway in June.

At the same time, the government is preparing to shell out billions for new fighter jets while the army has been clamouring for cash for new light and heavy trucks. Half of its current truck fleet has been parked because of age and maintenance costs.

The navy is also waiting to see how much money it will get for new surface warships, which are slated for construction at the same time the submarine life extension would take place. The budget was previously set at $26 billion for up to 15 vessels, but recent estimates have put the cost much higher.

Meanwhile, Sajjan said the government is looking at information submitted by various fighter jet companies at the end of July, but would not say when a decision will be made on replacing the aging CF-18s.

"Our staff is actually crunching through a lot of that data just now," he said. "I want to make sure to give them the opportunity to synthesize that information.

"I want to make sure we have all the right information so that we can pick the right process. This decision will be coming in months, and not longer than that."