A British tree thought to be up to 5,000 years old has started to change sex, a "rare and unusual" phenomenon not fully understood by scientists, a botanist said Monday.

The Fortingall Yew, in Perthshire, central Scotland has for hundreds of years been recorded as male, but has recently begun sprouting berries, suggesting that at least part of the tree is changing gender.

"It's a rare occurence ... rare and unusual and not fully understood," said Max Coleman of Royal Botanic Garden Edinburgh, who spotted the berries.

"It's thought that there's a shift in the balance of hormone-like compounds that will cause this sex-change. One of the things that might be triggering it is environmental stress."

However, the tree appears healthy, Coleman said, adding that it would now be closely observed for further changes.

The gnarled tree is situated in an old church yard, and is difficult to age because its heartwood -- the wood in the centre of the tree -- has long rotted away.

Scientists have been able to estimate by comparing its current size with measurements taken in the 1700s.