The UK military has signed a £15.4 million ($18.7 million) contract to buy a crewless submarine the length of a London double-decker bus, the government has announced. 

The 40-foot long, 17-tonne vessel — named Cetus after a mythological sea monster — is to be delivered to the navy by 2024 and will be the first crewless submarine in the British fleet, the Telegraph reports. It will be used to protect critical national infrastructure and monitor sub-sea activity.

The Ministry of Defence says that Cetus can cover 1,000 miles of the ocean floor in a single mission and has a maximum operational depth that will exceed that of the current submarine fleet. It will be the largest and most complex crewless submersible operated by a European navy, said a government press release.

First Sea Lord Admiral Sir Ben Key has said: "This Extra Large Autonomous Underwater Vehicle is a capability step-change in our mission to dominate the underwater battlespace."

The news of the new British vessel comes as the West considers the threat posed by the mysterious damage to the Nord Stream pipeline — a series of pipes that carries gas from Russia to Europe — in September. Russian saboteurs were suspected of being responsible for the Nord Stream leak.

John Brennan, the former CIA director, told CNN it was an "act of sabotage" and named Russia a suspect. At the same time, a British defense source said it was likely that the leak was caused by a pre-planned attack using remotely-detonated explosives.

The UK's need to protect its underwater space was highlighted in October when remote islands off the coast of Scotland were mysteriously cut off from the outside world, with phones and internet connections not working after damage to an undersea cable, Insider's Sinead Baker reported at the time. 

Celebrating the news of the new Navy vessel, Defense Secretary Ben Wallace said: "To meet the growing threats to our underwater infrastructure, the Royal Navy needs to be ahead of the competition with cutting-edge capabilities. We have the right equipment to protect the security of the UK and our Allies."