A medieval barn described by the poet John Betjeman as the "cathedral of Middlesex" has been rescued from decay and neglect for the nation, English Heritage said today.
Grade I-listed Harmondsworth Barn in west London joins the likes of Stonehenge, Osborne House and parts of Hadrian's Wall in the national collection of historic sites and monuments under the guardianship of English Heritage.
Built by Winchester College in 1426, the barn would have been used to store grain from the surrounding manor, owned by the Bishop of Winchester, with profits from the produce used to pay for the school.
The structure resembles the nave of a large church, standing at nearly 60 metres (200ft) long, 12 metres (40ft) wide, and 11 metres (36ft) high, with 13 huge oak trusses resting on stone blocks holding up the roof.
While it has had some repairs over the years, most recently by English Heritage to make it weather-proof and keep out pigeons, the structure is largely as it was built, with the timber and stones still bearing original carpenter and mason marks.
The oak-framed barn, which the heritage agency said ranks alongside the Houses of Parliament and Buckingham Palace for its historic value, was used up until the 1970s but fell into disrepair in the ownership of an offshore company which had bought it in 2006.
It is thought the purchase was a speculative one, as the barn stands just metres from where Heathrow's third runway - had it gone ahead - would have been built.
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.
Michael Dunn, historic buildings inspector for English Heritage, 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.
Simon Thurley, chief executive of English Heritage, said: "Harmondsworth Barn is one of the greatest medieval buildings in Britain, built by the same skilled carpenters who worked on our magnificent medieval cathedrals.
"Its rescue is at the heart of what English Heritage does - protecting this nation's architectural treasures and helping people discover our national story through them.
"We will complete the repair of this masterpiece and, working with local people, will open it to the public to enjoy."
A local group, the Friends of the Great Barn at Harmondsworth, formed around six years ago and have been dedicated to preserving the building, researching its history and keeping up the interest in its future, opening it each year to around 400-500 visitors during the Open House weekend.
The barn will now be open for free two Sundays a month between April and October, staffed by volunteers, with plans to open it every Sunday from next year.
Phil Rumsey, chairman of the group, said: "After working to save the barn over the last six years, it is wonderful that English Heritage have rescued this much-loved building. It will provide a great lift to the community."