Increased numbers of visitors and social distancing are eroding sensitive parts of the countryside, conservationists fear.
More of us have been taking to the outdoors for fresh air and exercise during the pandemic.
But walkers who step off paths to avoid coming too close to each other are trampling on the surrounding land.
One area the trust says has been ""significantly affected" is Dunwich Heath in Suffolk, along with areas of the Lake District.
Norfolk beauty spots such as Cley and Snettisham appear to be weathering any extra visitors.
Lloyd James, ranger at the site, said: “At Dunwich Heath on the Suffolk coast, the sandy soil has been eroded and the path has significantly widened in recent months, as more people move off designated paths to socially distance.
"This is particularly noticeable in an area of acid grassland adjacent to the cliff path, which visitors regularly use to explore the coastal site. In some areas, narrow paths of 50cm have now doubled in width."
Mr James said the area was particularly sensitive as it was home to nesting skylarks, numbers of which have declined in recent years. Rangers have installed additional signage and fencing to prevent further disturbance.
He added: “The soil on Dunwich Heath is low nutrient sandy soil, which is perfect for the acidic grasses and heath to thrive, but it’s easily damaged and takes a long time to recover. It only takes a few people to use a passing area before the vegetation is destroyed and becomes unrecoverable.
“Our top advice for walkers when encountering others is to walk single file. If you need to step aside to let others pass at a safe distance, please stop, wait and then return to the path before continuing your walk.”
With the upcoming Christmas holiday period likely to attract more people to get out and about in the countryside, the conservation charity is asking people to help minimise the effects of any further erosion.
Rob Rhodes, head of countryside management at the National Trust, said: “Many of our sites are currently seeing three times the usual number of visitors they would get on a busy summer’s day.
“However, landscapes are more susceptible to damage at this time of year due to the colder and wetter weather, and we want to help people understand how each and every one of us can play a role in looking after these beautiful places."
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