Knole House in Sevenoaks, Kent, is to receive £7.75m for repairs and conservation from the Heritage Lottery Fund.
The funding will also go towards the construction of new visitor facilities at the 15th century medieval palace, a former hunting lodge to Henry VIII.
The National Trust said that since acquiring Knole in 1946, it had faced expensive running battles with rain, damp, mould and insects, but the funding meant the future of the mansion and its treasures were now secure.
More top news
A lamp in the tower which houses Big Ben is to be switched off for the first time in more than 70 years.
Sir Bruce entertained audiences for more than seven decades.
Work in the tower will also pause over the bank holiday weekend.