Its rangers are reporting a substantial increase in the level of mess being dumped on the coast and countryside as lockdown eases.
And with more warm weather on the way, it is urging people not to use disposable barbecues because they risk starting a wildfire.
A National Trust spokesman said: “Despite recent rainfall, a record-breaking spring of sunshine has left many landscapes dry and created the perfect conditions for fires to ignite and quickly spread.”
Since the start of April, several large blazes have broken out on the Trust’s land all around the country, including one near the Devon coast that was started by a barbecue and required six fire engines and a police helicopter to extinguish.
Other fires seriously damaged wildlife habitats in Trust heathland and moorland.
People have flocked to the Sussex coast and countryside as lockdown has eased, leaving barbecues and rubbish in their wake.
Birling Gap in East Sussex was among the trust’s sites recording record numbers of visitors during the late May bank holiday.
The trust says many sites have noticed increased litter, which not only blights the landscape and poses a threat to wildlife but can fuel wildfires.
Ben McCarthy, head of nature conservation at the National Trust, said: “We know that people have missed the outdoors and open spaces these past few months – and we’re really pleased to be welcoming them back.
“But we’re urging people not to bring barbecues to the countryside or the coast. They can lead to real problems, particularly after such little rain in April and May.
“Many areas of land are still very dry and all it takes is a single spark from a barbecue or a dropped cigarette to cause a serious fire.
“Fires like these undermine our work to care for nature and respond to the climate emergency, which are priorities for the National Trust. Our local teams and the fire services are working hard to keep the countryside and coast safe for everyone, but resources are stretched.
“Please think of others; think of the wildlife; think of our emergency services; and don’t bring barbecues to the beach or countryside.”
Incidences of UK wildfires are increasing in number and severity, in part due to changing weather patterns.
National Trust lead ranger Katie Archer says: “Slindon is famed for its woodland, and the reason many come to visit. However, it is these very woods that are so vulnerable to fire. The countryside is tinder dry at the moment and a stray cigarette butt, barbecue or campfire could quickly spread into a devastating fire travelling both underground and over ground.
“Fires in the woods, deliberate or otherwise need to be reported immediately to the fire brigade. Please help us look after our countryside by reporting a fire immediately if you see one”.