A huge piece of a nuclear submarine has been moved into dock in the Lake District, as the UK continues to build a replacement for the current Trident vessels.

Built at a cost of £7.75bn per unit, four Dreadnought submarines will replace the current Vanguard ships which constantly patrol the seas carrying ballistic missiles equipped with thermonuclear warheads.

The Trident system is designed to act as a deterrent to other nations with nuclear capability, as a first strike on the UK would not eliminate the ability to strike back given some of the country's weapons are continuously at sea.

The concept of a nuclear deterrent is based on the doctrine of Mutual Assured Destruction (MAD), which holds that no country possession the weapons of mass destruction would use them against another nuclear-armed state because both would be destroyed the fighting.

The Herald:

At least one Vanguard submarine is kept on patrol at all times, carrying up to 16 of more than 100 nuclear missiles in stock. Missions are operated out of Faslane naval base.

Anti-nuclear campaigners maintain that the existence of the weapons is a danger to human life on the planet, and can point to a number of close calls since the USSR became the second country to enter the atomic age in 1949.

In 1961 a B-52 bomber carrying a nuclear weapon broke up in mid-air over North Carolina, with only a single switch preventing the bomb from detonating.

In 1983, Stanislav Petrov of the Soviet Air Defences received a warning that six missiles had been launched at the USSR by the United States but he accurately deduced that a malfunction had caused a false alarm and, against protocol, did not inform Moscow and thus prevented a potential retaliatory strike.

Replacement of the current submarines was approved by the House of Commons in 2016 despite opposition from anti-nuclear groups and campaigners concerned about the cost.

Three of the subs are currently under construction by BAE Systems, with the longest ever 'mega-unit' so far moved into the company's Devonshire dock hall in Barrow-on-Furness on Thursday.

Images make apparent the vast size of the structure, which towers above terraced houses.

At more than 20m, it was the longest submarine unit to move by road since the fore-end mega-unit moved to the DDH for the final Vanguard boat in the mid-1990s.