The Royal couple toured the house and its gardens
The Prince of Wales and the Duchess of Cornwall have visited a stately home in Northumberland to support a campaign to save it for the nation.
The National Trust needs £6.3m to buy Seaton Delaval Hall. It has raised £3m, mainly from public donations.
A number of public sector funding applications have also been made.
The Royal couple toured the Grade I-listed house and its gardens on Saturday, and thanked local supporters for their work.
Afterwards, Prince Charles paid tribute to the National Trust and everyone who worked on the project.
"Thank God an organisation like this exists in this country," he said.
"It is a great tribute that you have got so far with this."
Seaton Delaval was built between 1718 and 1731 by Sir John Vanbrugh, architect of Blenheim Palace and Castle Howard.
It is widely believed to be the finest work of the English Baroque and one of the most important historic houses in Britain.