‘My top-secret journey aboard one of many UK’s Vanguard submarines – here is what it is like’

The Mirror was given the rare chance to accompany Prime Minister Keir Starmer to board a submarine on “Day Zero”, its return to UK waters after a record 206-day deployment

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The whereabouts of Britain’s nuclear-armed submarines are one of the nation’s most closely guarded secrets.

One of the four Vanguard-Class submarines that carry Britain’s nuclear deterrent continuously patrols the seas, ready to fire Trident missiles on the Prime Minister’s command.

Only four people aboard know its route, and its location is kept secret for the length of its deployment.

It stays silent as it makes its slow, stealthy passage deep beneath the waves, receiving only incoming communications to allow it to stay hidden from hostile powers.

The Mirror was given the rare chance to accompany the Prime Minister to board a submarine on “Day Zero”, its return to UK waters after a record 206-day deployment.

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The trip was so secret that I referred to it only as “story X” in advance. Just a few colleagues knew where I was going.

In the early hours of the morning, I joined the PM on a choppy boat ride through the pitch dark to board the vessel, which we have agreed not to name for security reasons.

As the sun rose over the Scottish coastline, the surfaced vessel suddenly loomed into view, its hulking grey mass clear against the skyline, surrounded by a ring of boats manned by military police and armed Royal Marine commandos.

We approached slowly, waiting for the go-ahead before pulling up alongside where a line of Royal Navy submariners stood on the top, soaking up the daylight and fresh air after months below the waves.

I walked tentatively across the gangway onto the casing, before being escorted to the hatch, where I steeled myself to clamber down a steep ladder into the bowels of the vessel.

The first thing that hit me was the smell, a strange mix of oil and salt. A strong smell of fish, which occurs when the hatch first opens, had mostly dissipated, but can linger on people’s clothes for days after.

Through a disorientating maze of rooms and equipment lit by artificial light, we were shown the crucial parts of this cornerstone of Britain’s defence, from the communications hub to the control room, lit-up with screens and blinking buttons.

At one point, I realised I’d walked over where the nuclear reactor sits – but was reassured it was perfectly safe.

One room is decked out with advanced sonar systems, which flags up other vessels but also whales, dolphins and other marine life, unaware of the submarine gliding silently.

In one corner stood a safe containing a letter of last resort – instructions from the Prime Minister on what to do if an enemy nuclear strike has destroyed the British government.

Soon after entering Downing Street, Mr Starmer wrote these top-secret instructions by hand, which were delivered sealed and unread to the commanders of all four Vanguard submarines.

That responsibility clearly weighs heavily on the PM, who told me he found meeting the crew “humbling”.

The 160-strong crew works in shifts, often six hours on, six hours off for months, without seeing daylight. People are given SAD lamps and vitamin D supplements to cope without seeing the sun for so long.

Meals, I learned, are the main way to mark how time is passing. Despite their lengthy voyages, the cooks run a tight ship, serving up food every six hours – with Sunday roasts, steak, curry, and fish and chips all plated up on a regimented schedule.

To stay fit, there are running machines, weights and rowing machines dotted around. I was astonished to find the main place to exercise is a corridor flanked by missile casings.

Football scores are printed out and put up on the notice boards, with highly competitive sweepstakes running.

There’s even someone who cuts everyone’s hair, we were told by a group of sailors all sporting fresh trims for their return home.

Everyone is on top of each other all the time, sleeping in 9 or 18-bunk rooms, with only a locker for their stuff.

But the atmosphere creates tight-knit bonds as this underwater community is largely cut off from the outside world.

Submariners get a short message each week from their families – but they can’t reply, meaning their loved ones can go months without any contact.

One man told the PM that he had learned that he had become a father while he had been on patrol, and he was about to meet his son for the first time.

Others learn that someone they love has died while they are on patrol, a sign of the intense sacrifice made by the crew and their families during their lengthy deployments.