The Royal Navy is due to test fire a nuclear weapon within days. Officials issued a warning to shipping as nuclear submarine HMS Vanguard sailed into the Atlantic this week.
It is the first time the UK has test-fired a missile since a botched launch in 2016.
And it follows a drumbeat of warnings that the world is careering to World War Three.
The Sun understands the $4bn sub is scheduled to test fire an unarmed missile after completing a seven-year refit in Plymouth.
The tests are the final hurdle for HMS Vanguard to re-enter service as part of the UK’s nuclear deterrent fleet.
The 30-year-old submarine was pictured sailing from Port Canavarel in Florida on Tuesday morning.
The Navy hailed the doomsday vessel as a 491ft “colossus” that can patrol undetected for months at a time.
She can carry up to 16 Trident 2 D5 missiles, each armed with multiple British-made warheads that are each more than 20 times more powerful than the weapons dropped in World War Two on Hiroshima and Nagasaki in Japan.
The sub is expected to launch a single unarmed missile from 90km off the US east coast.
The US National Geospatial Intelligence Agency issued a warning to shipping that plotted the missile’s expected course to an impact in the mid-Atlantic.
The missile is due travel some 6,000 km before ditching into the sea between Brazil and West Africa.
The “hazardous operations” warning also plots areas much closer to the launch site where debris is expected to fall as parts of the 60-tonne missile are burnt out and discarded.
The US-made 44ft Trident missiles are designed to blast to the edge of space and track their position against the stars before re-entering the atmosphere and plummeting to earth.
Their maximum range is some 12,000km.
A Royal Navy source said its nuclear-armed submarines can each carry more explosive power than was dropped in the whole of the Second World War.
Officials routinely refuse to comment on nuclear and submarine missions.
However, the US warning remains in force from 9 pm on January 30 to 4 am on February 4.
HMS Vanguard left Plymouth last year after a £500 million overhaul that took three years longer than planned.
The Sun revealed that contractors who repairing HMS Vanguard bodged repairs in the reactor chamber by gluing broken bolts back together.
But the faults were detected in pre-mission checks.
In 2016 her sister sub HMS Vengeance had a Trident missile misfire after a similar refurb.
The intercontinental ballistic missile was due to fly 9,000km from near the coast of Florida to a target south east of Ascension Island.
But it veered dangerously of course and automatically self-destructed.
Former PM Theresa May was accused of covering up the malfunction ahead of a vote in parliament on renewing the UK’s nuclear deterrent.
