The equipment may be rudimentary and the technique suspect, but the pioneering spirit shines through. The earliest known footage of surfing off the chilly shores of the UK has been discovered in an attic and is being hailed as a national treasure.

Dating back to 1929, the footage shows Lewis Rosenberg, who was inspired to carve an 8ft (2.4-metre) board out of balsa wood after seeing film of surfers in Australia.

He and three friends travelled from their homes in London to Newquay in north Cornwall and leapt into the waves.

The footage was passed on to Rosenberg's daughter, Sue Clamp, who kept it in her attic until recently rediscovering it and handing it over to the Museum of British Surfing in Braunton, north Devon.

Peter Robinson, the founder of the museum, said: "When Sue visited one of our exhibitions and told us the family had film of surfing exploits on a wooden longboard in the late 20s we were totally blown away.

"We took the reels of fragile 9.5mm stock to the local film archive for them to be preserved and transferred to digital tape ... It was only then we realised just how special this film is. It is a national treasure as it shows the earliest recorded footage of surfing in Britain.

"We knew that belly boarding was happening at this time but this film is very significant. Lewis and his friends appear to have seen standing up surfing on a newsreel from Australia and just thought: 'We would like to have a go at that.'

"He was a highly inventive man. Not only did he create a surfboard with no references to draw from, but he also created a waterproof cover for his camera and actually took it out on the water. This has changed how the surfing community view their history."

Rosenberg, who was in his 20s at the time, travelled to Newquay with his friends Harry Rochlen and brothers Fred and Ben Elvey. They recorded their exploits on one of the first home movie cameras available, which did not have sound.

The British surfing legend and author Roger Mansfield, who has recently completed The Surfing Tribe, a book on the history of British surfing, hailed Lewis, now dead, as a surfing pioneer.

He said: "Nobody in the surfing community was aware of this story before now. This is definitely the earliest footage unearthed in Britain of stand-up surfing.

"I was gobsmacked when I first saw it. The footage gives a real insight into life at that time. This also has an impact on surfing heritage globally."

Sue Clamp, of St Ives, Cambridgeshire, said: "I had no idea my father's surfing would turn out to be so special."