The move follows the closure of pubs, restaurants, cafes, gyms and leisure centres announced by the government on Friday.Director-General of the National Trust, Hilary McGrady, said that this was due to the number of people still visiting the parks.
She said that the charity are "no longer convinced" that people will maintain social distancing while visiting the parks, particularly on Mother's Day.
Ms McGrady said: "Despite our desire to keep our outdoor spaces open, the health and wellbeing of our staff, volunteers and visitors has to be our top priority.
"Having observed the numbers visiting our properties today and I am no longer convinced we can maintain social distancing over Mother’s Day when numbers are likely to grow, and beyond.
"We have now sadly taken the decision to close all of our parks and gardens, in addition to our houses, shops and cafes, to avoid crowding that puts social distancing at risk.
"We know that people are likely to need space and fresh air in the coming weeks and months and we will do all we can to provide access wherever possible."
She added: "Our countryside and coastal locations remain open with parking charges waived, but we encourage people to stay local and observe social distancing measures.
"Over the coming weeks our digital platforms - our website, social media feeds, podcasts and video - will become even more important, ensuring the places of nature, beauty and history that we care for on behalf of the nation can remain open for business virtually while we are temporarily closed.
"We will also be ramping up our efforts to help people connect with nature wherever they are and to find moments of joy in the world around them. We will be providing rich content and staying in touch with our members and followers throughout this time."
The parks and gardens are closed as of midnight on Saturday, March 21.
Information on which National Trust outdoor places are open can be found at www.nationaltrust.org.uk
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