A medieval barn in west London described by poet Sir John Betjeman as the "Cathedral of Middlesex" has been bought by English Heritage for £20,000.
Grade I-listed Harmondsworth Barn is ranked alongside the Houses of Parliament and Westminster Abbey for its historic value, the body said.
Built in 1426 by Winchester College, it has undergone some repairs, but remains largely as it was built.
English Heritage said it would save it from neglect and decay.
The barn joins Stonehenge and parts of Hadrian's Wall in English Heritage's national collection.
'As good as it gets'
The structure resembles the nave of a large church, standing at nearly 60m (200ft) long, 12m (40ft) wide, and 11m (36ft) high.
It has 13 huge oak trusses which rest on stone blocks holding up the roof and the timber and stones bear the original carpenter and mason marks.
The structure would have been used to store grain from the surrounding manor, owned by the Bishop of Winchester, with profits from the produce going to pay for the school.
The barn was used until the 1970s but fell into disrepair under the ownership of an offshore company which had bought it in 2006.
In 2009, English Heritage became concerned about the barn's deteriorating condition and issued an urgent works notice for emergency repairs to keep it water and wind-tight.
A dispute over payment for the emergency works led to English Heritage buying the barn for £20,000 at the end of last year.
Michael Dunn, historic buildings inspector for the heritage agency, said the building was the best preserved and largest surviving medieval timber barn in England.
"This is high status, this is the finest timber, and a very confident carpenter. This is as good as it gets," he said.
Staffed by volunteers, the barn will be open for free two Sundays a month between April and October, with plans to open it every Sunday from next year.