The north-south divide has been the butt of jokes in Britain for years, but research has shown the Watford Gap, which separates the country, was in fact established centuries ago when the Vikings invaded Britain.

According to the archaeologist Max Adams, who made the discovery while researching his new book, the Northamptonshire-Warwickshire boundary known as the Watford Gap is a geographic and cultural reality that can be traced back to the Viking age.

Adams was struck by the absence of Scandinavian placenames south-west of Watling Street, the Roman road that became the A5. “There might be one or two names, but I don’t think there are any, and there are certainly hundreds and hundreds north-east. Clearly the Scandinavian settlers stopped at Watling Street,” Adams said.

“I began to notice that all the rivers’ sources stop pretty much on the line of Watling Street. North-east of that line, all the rivers flow into the Irish Sea or the North Sea. South and west of it, they all flow into the Severn or the Thames.”

He added: “Roman engineers constructing the route between London [Londinium] and the important town of Wroxeter [Viroconium], in what is now Shropshire, chose this ancient line, and it became Watling Street. In the Viking period it became the boundary for a treaty between King Alfred and the Viking leader Guthrum. Connecting the West Midlands with the south-east, it runs through a narrow pass between hills, the Watford Gap.

“I’m not sure whether people on the north side of Watling Street immediately feel themselves different or whether that’s more of a southern joke. But clearly it’s a joke with a very old reality attached to it.

“These days, we’re unaware of which way rivers face and where they flow out to. It doesn’t make any odds to us. We just put bridges over them. But, for most of history, such things have mattered. Your natural trading routes are along rivers and all the medieval monastic estates used the rivers as their arteries of power. So clearly the geography of power has always mattered … Geographically, it slaps you in the face as soon as you figure it out.”

He explained that the Anglo-Saxon kings eventually fortified that line and made it a frontier in the early 10th-century reigns of Eadweard the Elder and his sister Æthelflæd: “So, in a sense, they reinforced the reality of that piece of geography and it seems to have been with us ever since.”

“In 1959, when the M1 was first built, the Watford Gap was its end point – the butt of north-south divide jokes ever since,” said Adams. The M1 service station’s unofficial status as the country’s dividing point was celebrated in 2009 with the unveiling of a new road sign, with one arrow pointing north and another pointing south. Previously called the Blue Boar, the service station became famous as an early-morning hangout for The Beatles and the Rolling Stones, among millions of travellers who were fed and watered there. Linguists have since identified it as the boundary between northern and southern English.

But boundaries are certainly blurred, Adams said: “We find it bizarre that, on the news last night, people were talking about Cheshire as the north … Routinely, politicians describe Hadrian’s Wall as if it was the border between England and Scotland. Well, there’s another 60 miles of England beyond Hadrian’s Wall.”

Adams has excavated widely in Britain and abroad, and he will include his research in a forthcoming book, titled Aelfred’s Britain: War and Peace in the Viking Age, to be published by Head of Zeus on 2 November. It is a companion volume to his previous early medieval histories, The King in the North and In the Land of Giants.

In the new book, he notes that, before the second decade of the 10th century was out, new fortresses or burhs were constructed at 19 sites strung out on a broad line between the Thames and the Mersey, unmistakable in their offensive purpose. That line roughly follows Watling Street.

“It has an ancient and continuing geographic distinction, barely noticed by today’s midlanders. Broadly speaking, to the north-­east all the rivers flow into the Wash or North Sea on the east side, or the Irish Sea on the west. To the south and west every river drains into either Severn or Thames. This is England’s natural fault line, its continental divide: the watershed that divided and divides north from south (epitomised by the famous Watford Gap, on the A5/M1 north-east of Daventry); and I have no doubt that Scandinavian armies and settlers knew its imperatives.”