When the sleek, beautiful silhouette of Roskilde 6 appeared on the horizon, 1,000 years ago, it was very bad news. The ship was part of a fleet carrying an army of hungry, thirsty warriors, muscles toned by rowing and sailing across the North Sea; a war machine like nothing else in 11th-century Europe, its arrival meant disaster was imminent.

Now the ship’s timbers are slowly drying out in giant steel tanks at the Danish national museum’s conservation centre at Brede outside Copenhagen, and will soon again head across the North Sea – to be a star attraction at an exhibition in the British Museum.

The largest Viking warship ever found, it was discovered by chance in 1996 at Roskilde. It is estimated that building it would have taken up to 30,000 hours of skilled work, plus the labour of felling trees and hauling materials. At just over 36 metres, it was four metres longer than Henry VIII’s flagship Mary Rose built 500 years later, and six metres longer than the Viking ship spectacularly recreated as Sea Stallion, which sailed from Scandinavia around Scotland to Dublin in 2007.

“This ship was a troop carrier,” said Gareth Williams of the British Museum. It was built some time after 1025 when the oak trees were felled, and held 100 warriors taking turns on 39 pairs of oars if there was not enough wind to fill the square woollen sail. They would have been packed in tightly, sleeping as they could between the seats, with little room for supplies except a minimal amount of fresh water – or ale or mead, which would not have gone stale as fast – and dried salt mutton.

It would have been an uncomfortable journey, but short: they did not need to carry much as their ship could move startlingly fast – Sea Stallion managed an average speed of 5.5 knots, and a top speed of 20 knots. Once they landed, the warriors could forage with ruthless efficiency, as many a coastal community or wealthy monastery discovered.

The ship would probably not have come alone. “There are records in the annals of fleets of hundreds of ships,” Williams said. “So you could be talking about an army of up to 10,000 men suddenly landing on your coast, highly trained, fit, capable of moving very fast on water or land.” Such luxury ships were fabulously expensive to build and a devastating display of power, Williams said.

The dates suggest Roskilde 6 may have been built for King Canute, who according to legend set his throne in the path of the incoming tide, to prove to his courtiers that even a monarch could not control the force of nature. At the time the Vikings were consolidating their power from temporary raiders to permanent invaders.

With all the original timbers fitted into a steel frame that will recreate its full length and form, the ship will be the centrepiece of Viking, an exhibition opening at the Danish national museum in June, before being transported to London to launch the British Museum’s new exhibition space in 2014. It will travel in two containers, by freighter and lorry.

Accident

The vessel was found by accident when an extension was being built to the Roskilde ship museum in Denmark, itself built to hold an earlier find of Viking ships that had been deliberately sunk to narrow the fjord and protect the approach to the town, the old royal capital of Denmark.

In 1996 archaeologists watching the construction work discovered huge timbers turned up in the new foundations, some already chopped in half by the piling. It proved to be a treasure trove of nine ships, of which Roskilde 6, almost half of which was recovered, was the most spectacular.

The timbers stayed in storage while the museum worked out what to do with the unexpected addition to its collection, until the exhibition provided the opportunity for full conservation.

The original Roskilde ships are spectacularly displayed in a purpose-built ship hall, but could never travel: the timbers look solid but might shatter like glass. When excavated, the sodden timbers of Roskilde 6 would have disintegrated into a heap of dust if left exposed to air. National museum conservator Kristiane Straetkvern managed the project, which has been drying timbers up to 10 metres long far more slowly than the older techniques, then replacing the lost moisture with synthetic resin, leaving them lighter but stable.

It was a nervous moment for her when some completed timbers were test assembled, each resting in a felt lined individually laser-cut support, in a frame that bolts together like a giant Meccano set, but that dismantles into hundreds of components for travelling.

The exhibition will display finds from across Scandinavia and from deep into the countries they penetrated wherever a river could carry their shallow draft ships – as far inland as Lichfield in England, deep into Russia, to Byzantium in the east, where Vikings fought as mercenaries on both sides, and beyond. Objects from 12 countries, including many recent finds, will demonstrate that Vikings were traders, farmers, fishermen, and superb craft workers in timber, bone and metal.

However the most spectacular single artefact will be the ship, a potent witness that the Vikings were also dreaded raiders.

The Roskilde team are now experts on recreating ancient ships, regularly commissioned to build them. One day they hope to recreate a full-size, ocean-going replica Roskilde 6, and send it across the sea to awe rather than to terrorise the coasts of the British Isles.