The Hill House in Helensburgh was built by the renowned architect and designer as a "home for the future” between 1902 and 1904, for the publisher Walter Blackie.

Mackintosh wanted to dispense with fussy Victorian and Edwardian external detailing in favour of a shockingly plain exterior, and made use of a new material, Portland cement, to form a smooth render.

The result was admired and criticised in equal measure at the time, but has been allowing water to soak into the walls from the day it was applied.

Decades of wind and rain have since saturated the structure, threatening the long-term survival of Mackintosh’s “domestic masterpiece”, and the bespoke interior that he and his wife Margaret MacDonald created for their client.

The National Trust for Scotland, which owns the property in Helensburgh, now plans to put a transparent “cage”, designed by architects Carmody Groarke, around the building to keep the elements out.