Flooding, wildfires and extreme weather threaten the future of nearly three-quarters of sites managed by the National Trust, a new report says.
The charity says climate change is "the single biggest threat" facing its 28,500 historic homes, 250,000 hectares of land and 780 miles of coastline.
In Monday's report, the trust called on the UK government to do more to help organisations adapt to climate change.
The government said it had a five-year plan to boost the country's resilience.
Patrick Begg, the trust's natural resources director, said that climate change demanded "urgent and unswerving attention" and presented "the single biggest threat to the places in our care".
The trust is monitoring the climate change threats posed to its stately homes, museum collections, parks, gardens and land holdings by mapping current extreme weather events, such as downpours, flooding, drought and wildfires.
It then uses the data on its 'hazard map' to predict the threats posed to its sites under a "worst-case scenario" in which greenhouse gas emissions, which trap heat in the atmosphere, continue at their current rate.
The charity says planning for the worst will help it identify vulnerable sites across England, Wales and Northern Ireland and use knowledge gained on the ground by local teams to adapt to the impacts of climate change.
When the map was launched in 2021, it estimated that the number of National Trust sites facing a high level of threat from issues such as coastal erosion, extreme heat and flooding could rise from 5% to 17% over the next 40 years.
But in Monday's report - "A Climate for Change" - the trust now estimates that 71% of its sites could be at medium or high risk of being impacted by climate change-related extreme weather events by 2060.
In response, the conservation body is ramping up its work on climate adaptation, spending millions to repair and protect some sites but also having to decide where money could be better spent.