One bad winter storm could be enough to topple one of the oldest windmills in the UK, conservationists fear.
Bourn Windmill, near Cambourne, Cambridgeshire, is one of only about 50 surviving "post mills" in the country, and dates from the 1600s.
The Grade 1 listed Ancient Monument is at risk of collapse after decay was discovered in its supporting beams.
James Littlewood, of Cambridge Past, Present and Future (PPF) said it "tells a story of our pre-industrial history".
A campaign has been launched to raise funds to repair and maintain the building.
Unlike a fixed windmill, Bourn Mill can be manually rotated on a central wooden post - allowing the sails to be turned directly into the wind.
There are five remaining post mills in Cambridgeshire alone, including one at Great Gransden.
The mill is currently shut to the public and will remain closed while the money is raised to fully secure, repair and restore the decaying timbers, starting with a grant of £23,250 from Historic England.
But campaigners estimate the final cost of its repair could exceed another £50,000.
"Well-meaning" cement repairs in the 1980s enabled rainwater to infiltrate the ancient crossbeams, investigations revealed.
"This is a really important local and national piece of heritage and is a design that is largely unchanged since the 13th Century," said Mr Littlewood.
"Historic England experts found the rot was worse than we feared, so the building is potentially structurally unsound and could collapse."
He added: "I'm going to have sleepless nights through the winter if there's a big storm. We need to get it propped and scaffolded soon, to stop it falling down."
Archives show the mill was bought by a Thomas Cook in 1636, but Cambridge PPF believes the building is much older.
Historic England will now instruct a dendrochronologist to formally date the mill.
Mr Littlewood said he hoped the two-phase project of securing and restoring the building will see it reopen by May 2022.