The Ministry of Defence has agreed a new £1.4 billion contract for the Royal Navy's new attack submarine.
Agamemnon is the sixth out of seven in the Astute class fleet and will protect the UK's nuclear deterrent and new aircraft carriers.
The class of submarines are being built by BAE Systems.
Construction of the 7,400 tonne, 97-metre long Agamemnon began in 2012, alongside Anson, the fifth boat of the fleet, and the currently unnamed 'Boat 7'.
Their sister submarines, HMS Astute, Ambush and Artful are already in service with the Royal Navy on global operations.
Defence Secretary, Sir Michael Fallon, said: "This latest investment means we are well on our way to completing our fleet of Astute submarines.
"These are the most advanced submarines ever operated by the Royal Navy and are already providing unprecedented levels of stealth and attack capability across the world."
The Astute class submarines are able to travel the world completely submerged underwater, as it manufactures oxygen for its crew using seawater.
The fleet is replacing the Royal Navy's current Trafalgar class of submarines.