The National Trust estimates that the majority of its historic sites will be at risk of drought, flooding, wildfires and other climate change disasters by 2060.
Many properties are already experiencing first-hand the consequences of more frequent extreme weather events, a new report says.
The charity, which cares for 250,000 hectares of land, 780 miles of coastline and 220 gardens and parks, believes that more than 70% per cent of its 500 sites of could be at medium or high risk of climate hazards in 37 years.
It has called on the Government to introduce a climate resilience act to make climate change adaptation a legal requirement for public bodies.
The act could create national targets for climate change adaptation while imposing a statutory duty on public bodies to make adaptation a key factor in decision-making.
The trust said appointing a minister for climate adaptation in the Cabinet Office or Treasury would also be “pivotal”.
Patrick Begg, outdoors and natural resources director at the trust, said climate change presents “the single biggest threat” to the charity’s mission.
“It demands our urgent and unswerving attention, and we call on our partners and on governments across the UK to stand with us and to do more to confront the challenges we all face,” he said.
“Our responsibility spans hundreds of historic sites, buildings and some of the nation’s most-loved coastlines, rivers and countryside.
“These places are our national heritage and are treasured by people here in the UK and much further afield; last year we received 24 million visitors to our historic houses, gardens and estates.
“This is a serious obligation and we do not claim to have all the answers. But we do know that adapting to changing climate is essential if the trust is to live up to its founding purpose.”
The charity used its London site Ham House as an example of some of the work it is doing itself to combat the effects of climate change.
There is now a 10-25 per cent change of a heatwave every year at the 17th-century house - compared with a 10 per cent change a decade ago.
"We are exploring the restoration of the Victorian water-harvesting and irrigation system," the trust said, "The historic system is designed to collect rainfall from the house guttering in water chambers that can be used to water the garden.
"We’re also investing in better composting facilities for gardens to allow for more frequent mulching of soils. This will reduce moisture loss from new plantings and improve the moisture-holding capacity of soils."
It added that garden teams have been advised to keep water usage only for the most vulnerable or special plants " to explore changes to more drought-tolerant planting schemes".
The charity has also further developed its hazard map desktop tool to pinpoint the risk to its places from climate change.
Keith Jones, senior national consultant on climate change at the trust, said: “The hazard map flags the risk so that we can discuss with property teams what they are seeing in real terms, such as flooding, wildfire or overheating.
“By doing an ‘on-the-ground’ reality check with property teams, which essentially explores their experiences and detailed site knowledge, we can then assess the reality of these risks – whether they are great or small – and prepare accordingly.”
A Government spokesperson said the UK was the first country to legislate for net zero.
“In February, the Department for Energy Security and Net Zero was created to ensure climate change remains a key focus across Government,” the spokesperson said.
“Our third National Adaptation Programme sets out a robust five-year plan to strengthen infrastructure, promote a greener economy, and safeguard food production in the face of the climate challenges we face.”
