Volunteers Paul Nelson and Andrew Hunter with piano on Ben Nevis

Conservation volunteers from the John Muir Trust made the discovery during a clean-up operation on the 4,418ft Ben Nevis peak in the Highlands.

The piano has been broken up to make it easier to carry off.

The trust said how it came to be on Ben Nevis was a mystery but the mountain has been the scene of various stunts including cars being pushed to the top.

The piano was dug up from under a cairn during an operation by trust volunteers to reduce 100 cairns on the summit to 25.

Nigel Hawkins, trust director, said: "Our guys couldn't believe their eyes.

"At first they thought it was just the wooden casing - but then they saw the whole cast iron frame complete with strings."

We have a constant battle against litter being left on Britain's biggest highest mountain

Volunteer Sandy Maxwell

Mr Hawkins said the origins of the piano and how it got to the top of Ben Nevis remains a mystery.

But a McVities wholemeal biscuit wrapper also found with the instrument and dated best before December 1986 could be a clue.

He said: "The only thing that was missing was the keyboard - and that's another mystery. Maybe it's hidden somewhere else on the mountain."

Volunteer group organiser Sandy Maxwell, of Glasgow, said: "We are now trying to track down who took it there.

"We may even give them an outstanding invoice for 20 years storage of a piano under a cairn on Ben Nevis.

"We have a constant battle against litter being left on Britain's biggest highest mountain - but this elevates being a litter lout sky high into a completely different category."

He added: "We know cairns are often used to harbour rubbish but we never expected to find something like this."

One theory has been put forward as to how the piano got onto the mountain.

Former Radio 2 producer Paul Newman said he was walking up the mountain with his family in the summer of 1971 when they came upon a man carrying a piano up the mountain on behalf of a cancer charity.

We were coming down the mountain when we saw this surreal sight coming up through the clouds

Paul Newman

Former Radio 2 producer

Mr Newman said he remembered the piano as a half-sized instrument, strapped to the man's back using seat belts.

"He had a broad Scots accent and was wearing a kilt," he told BBC Scotland.

"We were coming down the mountain when we saw this surreal sight coming up through the clouds."

He said the man was leaving the piano on the mountain at night then returning the next day to carry it a bit further. They saw him on the third day as he neared the summit.

"He told us that when he got to the top he would spend the day playing it before pushing it over the edge."

It is thought that man could have been Highland Games athlete Kenny Campbell who said he carried a piano up the mountain on his back in 1971 although he claimed it never got to the top.

However, he did later carry an organ to the summit and played Scotland Brave while Norwegian climbers danced.

In 1911, a Model T Ford was brought to the summit in a stunt to promote the car.