DCNS Group has began M51 missile adaptation work for the last of three French ballistic nuclear submarines scheduled for the upgrade.

Work on the final submarine in the program, known as Le Téméraire, is set to continue for the next 18 months. The France-based defense contractor says the tasks will include extensive refit maintenance operations.

"This complex project illustrates the expertise of DCNS, as prime contractor and project supervisor. In addition to our own teams on the sites in Brest, Ruelle and elsewhere, we are coordinating the work of some 45 companies in the region around Brest and the West of France that have been called upon to carry out with us the 4 million hours of work required by the overhaul, all in compliance with our safety, deadlines and quality of work specifications," program manager Michel Drévillon said in a press release.

The M51 is a submarine-launched ballistic missile built by Airbus Defense and Space used by the French Navy since September 2010. Each of the missiles are capable of carrying six to ten independently targetable TN 75 thermonuclear warheads.

In addition to the adaptation, the maintenance work also includes upgrades for the vessels' navigation and combat systems.