The need for a new class of conventional submarine to replace the ageing Oberon class was made public in 1979, when the Type 2400 design was revealed.

Their operational role was reconnaissance in waters to shallow for SSN's , and to provide training. The initial requirement for twelve boats was cut to ten, and then the 1990 - 91 Defence Review cut the class to four. The cost of the four boats was £620 million (1987).

By this time, the Cold War was over, and their recon role was no longer needed, and in the hard financial climate of the day, it was decided that the Submarine Branch should be all nuclear. These fine boats were taken out of service in 1994, and laid up in Barrow Docks in the hope of finding a buyer.

Eventually, on July 2nd 1998, an agreement was reached between the governments of Great Britain and Canada for the lease and support of the four boats for a period of eight years at a cost of £254 million, with an option to purchase at a later date. The four boats have been successfully refitted and recommissioned into the Royal Canadian Navy as: