Legend has it that Marcus Carausius was enjoying a bath when a trusted aide entered, picked up a weapon and plunged it into his eye.

Not only did the bloody act end Carausius' life, but it also brought down the curtain on a man who was not only hugely influential in his time but whose legacy would forever shape Britain.

Were it not for him, a leading expert in Roman history claims, the Roman Empire may have left our island to its own devices more than 100 years earlier than it did.

A move which could have had a huge impact on our society and culture today.

So just why is a man who ruled the nation for longer than many prime ministers and thought to have masterminded the Saxon Shore Forts which still dot our coastline today, all but a footnote in history?

"Carausius is one of those great untold stories of British history which few academics will be aware of," explains Dr Simon Elliott, the Mereworth, near Maidstone, historian and broadcaster who has extensively researched the man he has dubbed the 'Pirate King' in his latest book.

"He's an astonishing individual in the narrative of British history, Roman Britain and, more broadly, because of his legacy.

Dr Simon Elliott talking about Carausius

"He put Britain back on the imperial map; he changed British history. And died for it."

So who was this man from Belgium who would provide such a pivotal moment in our nation's history?

"His story begins with Britain's first navy, the Classis Britannica," the author explains of the Roman fleet which patrolled the English Channel during its occupation, ferrying troops and supplies between us and mainland Europe.

"When you have a navy operating out of places like Dover or Richborough, you can control the open ocean - which in our case is the North Sea and the English Channel.

"When that disappears you're in real trouble because when the fleet's not there to protect that seaway then it's open to predating raiders. And the Classis Britannia disappears in the middle of the third century.

"The fleet goes and, hey presto, the North Sea is over-run by predating, Germanic raiders coming across to the east coast.

"So you get to the AD 280s and a Roman emperor called Maximian who is the emperor in the west, decides he has to do something about this as it's causing economic trouble not only for Britain but also the continent because the North Sea and English Channel is the umbilical cord which, economically, links Britain to the Roman Empire and it's been cut by this raiding.

"He appoints a really powerful admiral - someone who's the best chap he can turn to to take on this task. Carausius is an established military officer, based in Belgium, who's been helping secure the north western frontier of the Roman Empire.

"Once he's there, he does an amazing job. Such a good job, that someone in the Emperor's court whispers into Maximian's ear 'hold on a minute, something's going wrong here, he's so successful, I reckon he's pocketing what he's getting from these pirates he's capturing - I think he's on the make, in cahoots with the pirates'.

"And Maximian, who otherwise is a pretty good emperor, falls for it."

His solution is to draw up plans for Carausius to be executed. A rather extreme measure, but these, let us not forget, were the days when the Premier League of entertainment was watching people being fed to the lions. But his intended victim gets wind of the plans and decides the only route open to him is to claim power for himself and usurps his leaders and declares himself emperor of Britain and a chunk of Gaul in northern France. Suddenly he's created a position for himself within the Roman Empire while separating the island's direct control from Rome.

"Within that area," explains Simon Elliott, "he has three legions, he has the fleet he's just created to get rid of the pirates and potentially has another legion on the Continent.

"But he's also clever - and a bit of a PR man. He realises that Britain is the 'Wild West' of the Roman Empire - not that loved by the Romans and therefore it needs a bit of TLC.

"So he starts minting coins featuring himself as the Emperor which have far more silver content in them than any other coins minted on the continent.

"He has imagery on the back and words which reference great tales from the Roman past. He's trying to betray himself as the saviour of Roman values in this north western part of the Roman Empire. And to start off he thrives.

"Then Maximian tries to mount some kind of maritime invasion of Britain and fails. So the Roman imperial centre and Maximian can't come up with a strategy to defeat him so they have to put up with him."

Acutely aware of the dangers of losing face to the usurper, Maximian calls on Constantius Chlorus to go and defeat Carausius - a man now referred to as a 'pirate' within the corridors of power in Rome.

Chlorus was the head of the Pretorian Guard - the elite troops who guarded the Roman emperors.

"At the beginning of the AD 290s," explains the historian, "he starts a campaign against the north western rump which Carausius has still got in Gaul, and captures Boulogne (the French town in which Carausius had based his fleet to see off the Channel pirates).

"For Carausius, because he clearly relies on success, winning over the masses and military in Britain, suddenly he is on the back of a huge defeat. So big his number two, a chap called Allectus, who is one of these great figures in history we know nothing about, kills him.

"One story put about is that Allectus waits until Carausius is in his bath and then nips in there, picks a stylus off the table and stabs him in the eye with it - which is a bit of a gruesome way to go.

"Whatever happened, it's now AD 293 and Carausius is dead. So for the last three years of this breakaway northern empire that he created, his number two, Allectus, is in charge."

Thought to have been initiated by Carausius - but built under Allectus' direction - a series of forts are constructed along the south and Kent coasts - known today as the Saxon Short Forts. These include sites at Dover, Richborough and Lympne.

"Carausius was not daft," says the author, "he's a former military man - he knew they'd come for him, so he almost certainly started planning so he had a back up if his diplomatic initiatives failed.

"Whether he had begun them or not I'm not sure. But Allectus finishes them."

They do, however, fail miserably when put to the test.

"Chlorus then mounts a very successful, and clever, invasion of Britain from the continent," adds Simon Elliott. "He takes a third of his fleet along the north Kent coast along the Thames Estuary.

"Then, just off London, he stops. But he has ordered two-thirds of the fleet to land in the Solent, the soft-underbelly as it were, and they mount this very successful campaign all the way to London during which there is a battle and Allectus is killed."

As an aside, this victory was depicted on what has become known as the Arras Medallion. The golden coin, unearthed in 1922 near the French town from which it takes its name, shows a walled city. It's significant as that city is London and it is the first ever physical image of our capital city.

It brings to an end the reign of the "usurper's usurper" but changes our nation's history as a result.

Explains the historian: "I've always through Britain was the 'Wild West' of the Roman Empire. When Carausius revolted the Romans are getting fed up - what's the point, of all this trouble; you have the sea, all this usurpers and troops causing problems. But Maximian could not lose face in terms of losing Britain from the Roman Empire so he plays a game and mounts a fabulous campaign and Britain is dragged back into the Empire.

"Maximian has made so much effort to keep Britain in the Empire he can't then start thinking about letting it go. So it's like a recommitment."

Chlorous continues to have a long association with Britain - dying in York in AD 306. It is significant as his son would become Constantine the Great - the first Roman Emperor to convert to Christianity - a move which ushered in the religion here as well as across the Roman Empire.

"The fact Britain remained part of the Roman Empire for another 100 years," says the author, "is directly linked to Maximian making such an effort to get Britain back and all the major events leading up to Constantice the First becoming the first Christian emperor began here; culturally, relinking Britain to the rest of the Roman Empire.

"Only as you get into the fifth century do the Romans seriously considered letting Britain go again.

"I personally think, if Carausius hadn't usurped, the Romans may have just got bored with it and over the fourth century just completely let it go - which they did do with other parts of the Empire.

"If the Carausius event hadn't taken place and the Romans had left at the end of the third or beginning of the fourth century, Britain would not have participated in the cultural and economic development which happened on the continent which included things like the victory of Christianity over the classical religions, the development of the Roman economy, its legal system. We would have been cut adrift.

"Not being part of the wider Empire would have made a huge impact on Britain economically, politically and socially.

"Even when the British nobility do throw the Romans out at the beginning of the fifth century, within a year they're going back to the Romans saying 'can we come back in?' as they realise they've made a dreadful mistake."

Sounds a little familiar to the refrain of many today following Brexit.

Adds Simon Elliott, whose recent clip for history TV channel History Hit reviewing famous Roman scenes in movies has pulled in close to 700,000 views in just a month: "If you look at the type of jobs people did during the Roman period, 15-20% of those jobs were not involved in subsistence - so not involved in agriculture - they were constructing buildings, making pots, minting coins, serving in the military - to do that you must, through the rest of the population, make enough money and food to feed the 15-20%.

"That would have disappeared. Because that's exactly what happened when we left the Roman empire.

"All these jobs would have stopped and our economy would have changed.

"It's a genuinely big deal.

"Our experience of leaving the Roman Empire was so different to the continent that it has still changed the way we engage with the continent to this day."

Now his book, Roman Britain's Pirate King, takes a look at the remarkable forgotten chapter in Britain's history.

He adds: "Carausius dropped off the radar and is only spoken about very negatively in written histories. He's written out because he took on the imperial centre and lost.

"But he ran Britain for 10 years - longer than most prime ministers. For many of the kings and queens.

"If only for that he's a very important figure.

"Was he colluding with the pirates?

"The emperor clearly thought there was something going on. You could almost believe it - thinking here's an opportunity to make a few bob here and there - but I don't think he went the whole hog and sided with the people doing the raiding as he would have known if he'd been caught and arrested he'd have ended up in the arena being fed to the beasts.

"So I don't think he was daft enough to do that."

Roman Britain's Pirate King: Carausius, Constantius Chlorus and the Fourth Roman Invasion of Britain, by Simon Elliott, is out now in hardback and available through the likes of Amazon.