Hill House in Helensburgh is run by the National Trust for Scotland and the charity is looking to raise the cash to save it from “excessive water ingress”.

The building, which Brad Pitt visited when he was filming World War Z in Scotland four years ago, was built for publisher Walter Blackie in 1902 and is regarded as one of the architect’s most accomplished works.

However, Mackintosh didn’t use traditional weathering material and over the years damp has caused damage to the 113-year-old structure.

NTS’s director of conservation Terry Levinthal said the trust had agreed a strategy for repairing the building and is now launching formal fundraising plans and is asking for wealthy donors to step forward and help.

He said: “The Hill House is clearly a place of the highest cultural significance, and must be well looked after.

“Caring for our heritage can be very complex and this project is a prime example of that.

“The trust has been exploring how best to resolve the longstanding issues of water ingress. We have now concluded what our repair strategy is going to be, after considerable research investigation and consultation with stakeholders.

“This is time-consuming and, of course, the costs are significant too.

“We are in the early stages of fundraising in support of this major project to protect the Hill House, Mackintosh’s finest example of domestic design, for future generations.”