A giant "porous cage" is to be built around a crumbling architectural masterpiece while conservationists work to protect it in a £4m project.

The Hill House in Helensburgh, Argyll, was built as a "home for the future" by Charles Rennie Mackintosh between 1902 and 1904.

His choice of Portland cement for the render led to problems as it has allowed water to soak in for more than a century.

Decades of driving west coast rain have saturated the walls, threatening the long-term survival of the property.

It features bespoke interior designs that Mackintosh and his wife Margaret MacDonald created for his client, publisher Walter Blackie.

Famous visitors to the building include Brad Pitt and Angelina Jolie, who saw Hill House privately in August 2011 while Pitt was shooting the film World War Z.

Hill House owner the National Trust for Scotland (NTS) now plans to build a huge transparent cage, designed by architects Carmody Groarke, around the building to keep the elements out while conservationists come up with ways to protect it for the long term.

Simon Skinner, NTS chief executive, said: "As our president, Neil Oliver, put it, the Hill House is in danger of 'dissolving like an aspirin in a glass of water'.

"We are building what amounts to a shield around and above the Hill House to keep wind and rain out, and give the building a chance to dry.

"The structure is effectively a porous cage, albeit a beautifully designed one, that still allows some movement of air and a degree of moisture penetration - this is essential to ensure the walls do not dry out too quickly and crumble as a result."

The enclosure is expected to go up in 2018 and could be in place for a number of years.

NTS will launch one of the biggest fundraising drives in its history early in the new year to raise money for the £4m project.

The building will remain open to the public while conservationists are at work, and visitors will be able to climb stairs and gangways to gain a bird's eye view of Mackintosh's masterpiece and watch the restoration work as it progresses.