None of the Royal Navy's seven attack submarines are deployed on operations at the moment, according to reports, which potentially threatens the security of Britain's nuclear deterrent.

The Sun reported this morning that six of the seven boats are in maintenance – except for the seventh, HMS Astute, which is still undergoing sea trials.

Her sister Astute-class boat, HMS Ambush, is still undergoing repairs after ramming a civilian tanker in 2016 while hosting wannabe sub captains undergoing the Perisher submarine command course.

The main uses of attack submarines, as distinct from the four Vanguard-class boats that carry the UK's Trident nuclear deterrent, are twofold.

As their name suggests, attack submarines can be used to attack the enemy – or, in the modern world's uneasy peace with Russia, trail their submarines and naval vessels around the high seas, gathering vital intelligence on their sound signatures and electronic emissions for later analysis.

Attack submarines can also be used as escorts for other vessels, using their advanced sensors to sweep the seas for hostile ships such as Russia's infamous intelligence-gathering trawlers, as well as doing secret squirrel tasks such as dropping off Special Forces in remote shore locations. British attack boats also deploy on intelligence-gathering missions into the seas around Russia and have chased Russian submarines away from Britain's territorial waters.

With no attack submarines able to put to sea – if one believes The Sun – this leaves the UK's maritime defences perilously low. The attack submarine fleet is used to help cover the entry and exit of the Vanguard boats from their base on the Clyde, which is a long, straight and narrow approach that can easily be surveilled by Russian submarines. Indeed, that very thing happened at least once in 2014 – and has done on many more occasions since.

Respected defence journalist and editor of Warships International Fleet Review magazine Iain Ballantyne suggested, in 2015, that only two of the RN's attack boats are available for deployment at any one time.

He also suggested that the RN should stop buying hideously expensive nuclear submarines from BAE Systems and instead buy relatively cheap electric modern U-boats from Germany. The price tag for HMS Astute alone was more than £1.3bn at 2015 estimates, and in 2012 the Guardian claimed that the submarine was dogged by construction quality control problems.

The Ministry of Defence does not comment on submarine operations. ®