Ben Nevis 'not UK's tallest peak'

But before Munro baggers start poring over their maps, they should know that the summit of this mighty peak is nearly 1,000ft below sea level.

The massive, extinct volcano is some 40 miles across and about 1,800ft higher than Nevis, as measured recently by staff at the British Geological Survey.

Ken Hitchen, a marine and petroleum geologist, said the "seamount", which lies in Scottish territorial waters in the Rockall Trough to the north-west of Lewis, should be recognised as the highest mountain in the UK.

He claimed the system of counting from sea level made no sense, and said: "You are, in effect, assuming every mountain has its base exactly at sea level, not a foot above or a foot below. That's rubbish, isn't it? There's no difference between a mountain that is underwater or a mountain on the shore. If you wade into the sea up to your waist, you are still 6ft tall.

"But people tend to ignore everything underwater because they cannot see it. It loses importance because you cannot walk, climb or drive over it."

Mr Hitchen said it was difficult to define the base of any mountain, but under his system the record books should be substantially rewritten. "Everest [29,035ft above sea level] is not the world's highest mountain. Hawaii is about 32,000ft, but it's only the top 13,000ft or so that's above sea level," he said.

"If I had my way, I'd find a different way of defining the height of mountains. We need to define the base. Everest doesn't have its base at sea level. It's just like a little peak at the top of the Tibetan plateau."

Ben Nevis would, according to Mr Hitchen, lose about 200ft if measured from its true base above sea level, "which is quite a lot".

Rosemary Bank, which is believed to be named after a Royal Navy survey boat, called Rosemary after the herb, was created by a volcanic eruption between 56 million and 70 million years ago. There is a string of similar volcanic seamounts off the west coast of Scotland, including the 5,700ft Anton Dohrn.

Dave Morris, director of the Ramblers Association Scotland, was reluctant to accept Rosemary Bank's claim to fame.

"It is pretty interesting to know what the structure is under the sea. I think most people will be surprised how mountainous and undulating the seabed is. A lot of people think it's pretty flat," he said.

"But, for all practical purposes, most climbers and walkers will use sea level [to measure mountains]."

Nigel Hawkins, the director of the John Muir Trust, which owns Ben Nevis, suggested the mountain's true base was actually below sea level and might be considerably higher, rather than lower.

"Ben Nevis does rise from the sea and you could argue it is more than 4,400ft," he said. "When you are on the ben itself - and lucky enough to have a clear day - you can look down to the water and see that it is very much a mountain of the sea."

He suggested the news of Rosemary Bank's claim to fame might cause a problem for Munro baggers - people who try to climb every peak above 3,000ft.

"Are they going to have to get out the sub-aqua gear? Would you have to go down to the bottom and work your way up to the summit, or could you just land on the top? Knowing how devious Munro baggers can be, they'll come up with something," he said.