It has lain undiscovered on the Norfolk sandbanks for hundreds of years, since sinking during a maritime disaster that almost killed the heir to the throne.
Now, the wreck of HMS Gloucester has been found by amateur divers in what experts have described as the most significant maritime discovery since the Mary Rose.
The royal ship ran aground and sank in 1682 while carrying the future King James II, a Catholic heir poised to inherit the Protestant throne from King Charles II at a moment of acute political and religious crisis in Britain.
Two brothers, Julian and Lincoln Barnwell, found the warship after spending four years combing thousands of miles of seabed, before embarking on a painstaking, years-long identification process.
It took another five years for historians to confirm that the wreck was the Gloucester. It was split down the keel and remains of the hull are submerged in sand, complicating the excavation.
In 2012, the ship’s bell was used by the Receiver of Wreck and the Ministry of Defence to identify the vessel, as it had the distinctive date of 1681 engraved into it.
But the discovery has been kept secret for a decade.
The extensive work required to protect the security of an “at risk” site in international waters - the precise location of which will not be publicly disclosed - has meant it is only now that the discovery can be announced.
It will ensure the site can only be accessed by divers with proper licences to deter others from plundering it for valuable artefacts.
Researchers hope the wreckage will shed new light on a tragedy, which had major political ramifications and helped shape the troubled reign of King James II.
“Because of the circumstances of its sinking, this can be claimed as the single most significant historic maritime discovery since the raising of the Mary Rose in 1982,” said Prof Claire Jowitt, a world-leading authority on maritime cultural history and head of the accompanying research project at the University of East Anglia (UEA).
“The discovery promises to fundamentally change understanding of 17th-century social, maritime and political history.”
The remains of the ship, around 27 miles off the coast of Great Yarmouth, will help researchers understand the circumstances of the Gloucester’s sinking, which were bitterly disputed at the time.
Historians believe the behaviour of James Stuart, then the Duke of York, helped sow the seeds of rebellion among both a close adviser and the controller of the navy, who would later assist efforts to depose him.
The ship set sail from Portsmouth, picking up the Duke in Margate on his way to collect his heavily pregnant wife from Edinburgh, so the family could return to England. King Charles II, his elder brother, was in poor health and had no legitimate children.
At around 5.30am on May 6, the Gloucester ran aground on the treacherous Norfolk sandbanks following a dispute involving the Duke, a former Lord High Admiral, who insisted the pilot followed his course.
The Duke delayed abandoning ship to the last minute, needlessly costing the lives of an estimated 130 to 250 people who, because of protocol, could not abandon ship before royalty.
He accepted no responsibility for the sinking, instead blaming James Ayres, the pilot, who was court-martialed and imprisoned.
Prof Jowitt said: “James’s behaviour on the ship and afterwards likely changed how a lot of powerful individuals thought about him.
“A tragedy of considerable proportions in terms of loss of life, both privileged and ordinary, the full story of the Gloucester's last voyage and the impact of its aftermath needs re-telling, including its cultural and political importance, and legacy."
The sinking of the Gloucester was witnessed by Samuel Pepys, the diarist, who described in a letter to a friend how survivors and victims were picked up "half dead" from the water.
Reflecting on how much worse the disaster could have been, he wrote: “Had this fallen out but two hours sooner in the morning, or the yachts at the usual distance they had all the time before been, the Duke himself and every soul had perished.”
Many valuable artefacts have already been recovered, including a wine bottle bearing a glass seal with the crest of the Legge family, which was the forerunner to the US Stars and Stripes flag and belonged to the ancestors of George Washington.
The improbable mission that led to their recovery began in around 2003, when Lincoln Barnwell was flicking through a book on shipwrecks and read about HMS Gloucester.
He and his brother, printers by trade, had been passionate divers since their childhood and decided to see whether they could find the lost ship off the coast of their home county.
On the day they discovered the wreck, they had almost given up in unfavourable conditions - but decided to press ahead with the dive, having made the journey.
They began spending “every window of opportunity” on diving voyages that could last up to 18 hours a day, assisted by survey equipment that helped them narrow their focus.
However, it still took four years - and around 4,000 miles - for them to make their breakthrough in 2007.
“I was heading down the anchor line and about three-quarters of the way down, in really good water visibility, I started to see some shadows on the seabed and before you knew it, I was kneeling on the seabed literally in awe of the sight of these magnificent cannons,” Lincoln Barnwell, 51, said.
For the brothers, finally being able to speak publicly about their extraordinary work has proved to be a strange ordeal.
Julian Barnwell, 55, said: “I can’t believe it, it makes us nervous - for so long, we’ve had to keep it quiet, to protect and survey, we’ve been acting like the guardians, that’s how it feels."
A major exhibition will be staged in Norwich between February and July next year, produced through a partnership of the Barnwell brothers, UEA and the Norfolk Museums Service.
It will display finds from the wreck - including the bell that confirmed the ship’s identity - and share ongoing research.
