A Royal Navy submarine suffered a fire on board, Forces News has been told.
The fire on HMS Victorious "was in a self-contained module and was quickly extinguished", according to a Navy source.
A Royal Navy spokesperson said: "The Continuous At Sea Deterrent [CASD] is unaffected, but we do not comment on the detail of submarine operations."
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The Navy added that all seafaring personnel are qualified firefighters, helping ships and boats to respond quickly and effectively to emergencies.
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According to reports in the Sun newspaper, the Vanguard-class Victorious ditched a top-secret mission and rose to the surface to deal with the onboard fire.
The newspaper also reported that she was carrying Trident nuclear missiles when the blaze was dealt with.
Up to four British nuclear submarines, based at Scotland's HMNB Clyde, patrol global waters, carrying Trident thermonuclear warheads as part of the UK's CASD.
With an around-the-clock, at-sea deterrent, the UK is recognised as one of five nuclear states under a United Nations pact.
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Alongside the four other permanent council members – France, Russia, the US and China – the UK operates under the Nuclear Non-Proliferation Treaty (NPT).
The Vanguard-class vessels are due to be replaced by the incoming Dreadnought-class in the 2030s – set to be the largest submarines ever operated by the Royal Navy.
Four of the vessels are set to cost the UK more than £30bn to produce.
Following Nato policy, the UK uses minimal credible nuclear deterrence, but does not follow a policy of 'no-first use' – which would prevent launching a nuclear attack unless first coming under nuclear attack from another state.
It has vowed not to use or threaten a nuclear attack against any non-nuclear state within the NPT unless the agreement is breached.