A maintenance contract to keep some of Canadian air force's near-55-year-old Sea King helicopters flying has been extended to end of this year, federal documents reveal.

The cost to taxpayers will be $2.1 million, a spokesperson for Public Services and Procurement Canada (PSPC) confirmed in an email.

The authorization was granted under a long-term agreement signed five years ago, which has seen a total of up to $459 million spent to keep the aging choppers in the air, according to the documents and department officials.

Successive governments have struggled to replace the Sea Kings and the maintenance extension became necessary to "provide uninterrupted coverage to ensure that the CH-124 Sea Kings remain a safe, airworthy and capable aircraft until it is decommissioned and replaced by new helicopters," according to a Dec. 29, 2017 letter to the Treasury Board, requesting the extension.

The letter, signed by the deputy minister of PSPC, was obtained under access-to-information legislation.

The former Conservative government ordered the phased retirement of the Sea Kings, starting in 2015. The last one is expected to come out of active service in December.

The helicopters have been flying off the back of Canadian destroyers and frigates since the 1960s.

A CH-148 Cyclone maritime helicopter takes part in a training exercise at 12 Wing Shearwater in 2015. Earlier this year, the air force said it had taken delivery of eight Cyclones, with the rest slated to come by 2021. (Andrew Vaughan/Canadian Press)

Public Services spokesperson Rania Haddad said no further extensions are planned as the last of the 24 remaining helicopters are retired.

"A total of 15 helicopters, either currently flying or retired, have been made available for sale, and an additional three helicopters will be sold for disposal and recycling of materials," Haddad said.

While there have been a number of expressions of interest, Haddad refused to disclose which countries or organizations might be interested in the aircraft, citing privacy reasons.

There were reports last year that the government of India was considering a bid following Defence Minister Harjit Sajjan's trip to the country. But the documents show the informal discussions were not immediately followed up with a formal written offer.

Replacements slow to arrive

The Sea Kings are being replaced by CH-148 Cyclone helicopters. But the program has been painfully slow, as U.S.-based manufacturer Sikorsky worked out design kinks associated with converting the aircraft from civilian to military use.

The former Liberal government of Paul Martin signed a $5-billion contract with Sikorsky to buy 28 Cyclones, which were supposed to begin arriving to the air base in Shearwater, N.S., in 2008.

Repeated delays — brought on by design changes requested by the military, as well as technical problems — pushed the original projected delivery date back by years.

The first Cyclone arrived at Shearwater in 2011, but was only partially configured, and the contract became the subject of lengthy negotiations between the manufacturer and the federal government.

In 2010, the program was the target of the former auditor general Sheila Fraser, who complained about cost-overruns and delays.

Earlier this year, the air force said it had taken delivery of eight CH-148 Cyclones, with the rest slated to come by 2021.

And last month, the Canadian military showed off one of the new helicopters as it left for its first international deployment aboard HMCS Ville de Quebec.