The Major Oak, one of the UK's most iconic trees due to its vast age, size and links to the legend of Robin Hood, is believed by experts to have died.
The ancient oak is estimated to have lived in Sherwood Forest in Nottinghamshire for up to 1,200 years, and is considered one of Britain's biggest oak trees.
Conservationists worked to protect the tree, which attracted millions of visitors over its life, but in recent years the natural giant was in decline.
The Royal Society for the Protection of Birds (RSPB) - the conservation charity which manages the forest - added after the tree's first spring with no leaves this year, scientific experts said the Major Oak had died.
While it was difficult to determine the exact cause of the oak's demise, the RSPB said a combination of issues - including years of "well-intentioned structural intervention and huge amounts of human activity" around the tree - were thought to be "major contributors".
The charity said the effects of climate change - including recent heatwaves and droughts - had compounded challenges it would naturally be facing at its age.
The Major Oak will remain standing in the forest as a monument for people and wildlife, the RSPB confirmed.
Saplings from the tree have also been planted across the world.
Chloe Ryder, RSPB Sherwood Forest estates operations manager, said the cause of the tree's death was "complicated", but agreed that tourism had played a part by compacting the soil.
She said staff had done "everything they can" since taking over the site to keep it alive.
"It is of such importance to Sherwood Forest," she added.
"We now know that it's come too late, right at the end stages of its life.
"But there wasn't anything more that we could be done unless you invent a time machine, really."
Pyne added the tree would not immediately disappear, and as it decayed over the next few decades, would provide a home to many animals and plants.
Simon Parfey, a specialist in soil microbiology testing - who had been part of an expert team caring for the tree since 2021 - said the soil around the tree was "under far greater stress than anyone had initially realised".
"While the Major Oak team worked tirelessly to revive the environment around this iconic tree - and saw encouraging signs of life in some areas - the damage, it now seems, was already too deeply entrenched to fully reverse," he said.
Parfey said the lessons learned from the Major Oak would help to care for other ancient trees across Britain.
"The Major Oak's true legacy is no longer just in folklore, but in the future of conservation," he said.