BBC's Countryfile measured the ancient tree in a recent programme
Cold weather is believed to have caused one of Britain's oldest trees - known as the Great Oak at the Gates of the Dead - to split down the middle.
The 1,200-year-old oak, near Chirk, Wrexham, has a 34ft (10m) circumference trunk and is thought to date back to the reign of King Egbert in 802.
It is near the site of the Battle of Crogen in 1165 when the tree is thought to have been spared by King Henry II.
Local historian Mark Williams said he was "devastated".
Mr Williams and fellow historian Deryn Poppit discovered the damage on Tuesday.
He added: "It seems to be a victim of the very cold weather.
"The tree is on marshy ground in a basin with a stream running down nearby.
"With the stream overflowing because of melting snow, the water must have settled around the trunk and it looks as if this has caused it to split.
"It's a great shame that this has happened after the tree managed to survive for so long. We're really devastated."
The tree was spared when King Henry II had his men cut down the Ceiriog Woods in 1165 before he met - and was defeated - by Owain Gwynedd in the Battle of Crogen.
A plaque marking the battle was unveiled last year after Mr Williams and Mr Poppit campaigned to have the site officially recognised.
Last month the tree was featured on BBC1's Countryfile .
Mr Williams added: "Although some parts of the tree were rotten, some of it was still as strong as an oak.
"At some stage in its history, it appears to have been struck by lightning."
After hearing about the damage, Mike McKenna, director of the Chirk wood-based panel producer, Kronospan, has offered to try and preserve the oak.
He said he had asked a specialist firm of arboriculturalists to carry out an assessment, adding: "If there's something positive that can be done, we'll do our best to help."
Expanding ice at the tree's roots has caused it to split into two