OTTAWA -- The Trudeau government is planning to spend billions more on the navy's four wayward submarines to keep them operating into the 2030s.

The plan to extend the lives of the troubled vessels is included in the Liberals' new defence policy and comes following calls from senior naval officers to save the controversial ships from the scrap heap.

The actual price of the plan was not revealed in the policy document, which was released to much fanfare last week, and National Defence refused to provide a price tag following multiple requests.

That is despite assertions from the Liberal government that the defence policy was fully costed and following promises of full transparency when it came to the overall plan.

"Detailed costing will be provided in the Defence Investment Plan to be published in due course," National Defence spokesman Daniel Le Bouthillier said in an email.

Defence sources, however, have told The Canadian Press that keeping the submarines in the water for another decade will cost upwards of $2.5 billion.

Without upgrades, the first of the submarines will reach the end of its life in 2022, according to documents obtained last year through Access to Information, with the last retired in 2027.

Some have questioned the wisdom of spending more money on the four vessels, which have been plagued with problems since Canada bought them used from Britain in 1998.

While the Chretien government said at the time that it was getting a bargain by paying only $750 million, the ships have required constant repairs and upgrades just to make them seaworthy for a limited time.

And while a number of experts have called for Canada look to purchase new submarines, rather than upgrading the ones it has, others have said the country doesn't need such expensive vessels.

Defence Minister Harjit Sajjan this week emphasized the Liberal government's view, previously expressed by senior naval officials, that subs are necessary for protecting Canada's security and sovereignty.

"No other platform in the Canadian Armed Forces can do what a submarine can do," Sajjan said during an event in Halifax on Monday.

"No other platform has the stealth, the intelligence-gathering, surveillance and reconnaissance capability and the deterrence to potential adversaries that a sub does."

Sajjan added that the government decided upgrading the existing subs -- HMCS Chicoutimi, Victoria, Corner Brook and Windsor -- was more "prudent" than purchasing new vessels.

The Liberals promised in their defence policy to invest an additional $62 billion in the military over the next 20 years, which includes increasing annual defence spending by 70 per cent over the next decade.

A large chunk of that new money will end up going towards replacing the navy's 12 frigates and three recently retired destroyers with 15 new warships at a cost of between $56 billion and $60 billion.

Previous estimates had pegged the cost of those vessels at $26 billion.

The four submarines continue to generate headlines for the wrong reasons, with the most recent Thursday when HMCS Chicoutimi was hit by another naval vessel while docked at CFB Esquimalt in B.C.

But Rob Huebert, an expert on maritime security at the University of Calgary, said the other three have been involved in a variety of tasks and mission in recent months -- even if most Canadians don't realize it.

"The very nature of what they do means that (the military) can't talk about it," he said.

"They're actually exceeding what the navy was expecting them to do in terms of time at sea, interdiction of drugs and co-operation with the Americans. You can't talk about any of that, but it is occurring."

Rather than extend the lives of the submarines, Huebert said he would have liked to see the government start looking for replacements, but that wasn't possible given the huge costs of replacing the frigates.

"What we saw was the defence review was an intelligent decision to do what was necessary to lengthen the life of the subs while making sure the (new warships) are built," he said.