Plans to build a new "super-hospital" in west London face collapse in a row over its spiralling costs.
Proposed costs for the Paddington Health Campus started off at £350m but are thought to have soared to £800m.
The Royal Brompton & Harefield Trust has withdrawn its backing, saying it might now be too expensive.
The project's director said the row meant the future of the campus, as defined in its original brief, had been "put into question".
"And it means the closure of the project is now being considered," he said in a statement to BBC London.
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We believed all along that this scheme would not work
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The state-of-the-art specialist heart and lung hospital would have merged services from St Mary's, the Royal Brompton and Harefield hospitals on one site on Paddington Waterside in London.
Prime Minister Tony Blair once called PHC an "imaginative, extraordinary" project.
But documents obtained by BBC London under the Freedom of Information Act reveal that government officers expressed concerns that the project would prove unaffordable earlier this year.
A report from the Department of Health and the National Audit Office also raised concerns about the huge expense of the project.
It is believed that, including consultants' fees, about £10m has already been spent on the plans which were originally announced in 1997.
Campaigners who have battled to keep Harefield Hospital open greeted the news with delight.
Jean Brett, who chairs the Heart of Harefield campaign, said it was "a load of rubbish from day one".
"We believed all along that this scheme would not work," she said.
Harefield is world famous for being one of the best heart and lung centres in the world. Surgeons at the hospital have carried out more than 2,000 heart transplants and thousands of heart bypass operations.