What is the UK's longest river? It is a question that regularly pops up in pub quizzes and trivia books but the answer may not be clear cut.
Textbooks tell us the River Severn is the longest - at 220 miles (354km), and the River Thames slightly shorter at 215 miles (346km) long.
But what if the source of the mighty Thames, which snakes its way from Gloucestershire, through south-east England, to London and the Thames Estuary, was in the wrong place on the map?
The precise length of a river is not easy to calculate and depends on correctly identifying the source.
The Ordnance Survey (OS) map shows the official source of the Thames to be at Thames Head, near the village of Kemble in the Cotswolds, about 5 miles (8km) south-west of Cirencester.
However there is a popular belief among locals that Seven Springs near Cheltenham is the real source of the Thames.
If this was the case it would add a further 14 miles (22km) to the Thames, making it nine miles (14km) longer than the Severn.
Locals point out that Seven Springs is the source of the River Churn, which flows into the Thames at Cricklade, and Seven Springs is further from the mouth of the Thames than Thames Head.
"The Churn/Thames river may therefore be regarded as the longest natural river flow in the United Kingdom, beating its nearest rival, the River Severn, by some nine miles," said Duncan McGaw, vice chairman of Coberley Parish Council, in whose parish Seven Springs lies.
There are even two stone plaques set into the walls above the springs at Seven Springs, both bearing the Latin inscription, which is translated as "Here, O Father Thames, is your sevenfold source".
However there is also a plaque at Thames Head, which reads "This stone was placed here to mark the source of the River Thames".
And the debate is not a new one.
More than a century ago scholars were writing about the rival claims, and in February 1937 two local MPs, Walter Perkins and William Morrison, debated the issue in Parliament.
Ever since then it seems the source has been placed firmly on the map at Thames Head.
So which is correct? Who decides where the source of a river is, officially?
And could the official source be moved to Seven Springs? In theory, the answer seems to be "yes".
A spokeswoman for the Environment Agency said: "To the best of our knowledge, there is no legally designated source of a river, including the River Thames.
"The Environment Agency, and National Rivers Authority before us, have used the source identified on OS Maps, which is the historically accepted location at Thames Head."
Dick Mayon-White, a member of the River Thames Society, a charity which aims to protect and preserve the river, says his organisation goes along with the official Thames Head view.
"I'm not really sure there's a huge amount to be gained from trying to designate Seven Springs as the source unless or until people can organise some kind of pathway that walks the whole length of the River Churn up to Seven Springs," he said.
"I don't have any objection in principle, but it would upset a lot of people who've done work on describing the Thames Path and writing guides to the Thames Path.
"It's quite helpful from our point of view to have something that's officially decided, because otherwise it makes it very hard for us to decide what we're interested in and what we're responsible for."
So is the River Thames actually the UK's longest river? Or should we go with the flow and carry on saying the Severn holds the record? The debate continues.