A £3 million gift to the National Trust will help it conserve historic objects ranging from Samurai armour to a sixteenth century portrait with a secret, the charity has said.

The gift from American charity the Royal Oak Foundation, which raises support for the National Trust in the US, will help five years of conservation work and protect more than a hundred historic artworks, the Trust said.

It will fund work mainly based at the charity’s specialist conservation studio at Knole, in Kent.

Some of the first objects to benefit from the funding include an oil painting of Sir John Maitland, from Ham House, in London, which is in need of conservation work including removal of varnish and overpaint.

In 2017 X-ray analysis revealed what is believed to be an unfinished portrait of Mary Queen of Scots hidden beneath it, and conservators now hope to learn more about the painting through technical art historical investigations.

A suit of Japanese Samurai armour, one of a set of 27 bought by collector Charles Paget Wade for his home at Snowshill, Gloucestershire, which needs cleaning and treatment on the metalwork and textiles, will also benefit.

And a set of nine early 17th century carved, painted and partially gilded Italianate sgabello chairs from Petworth House, West Sussex, which need structural treatment and conservation of their highly decorative surfaces, are among the first objects that will benefit from the funding.

Dr Tarnya Cooper, the Trust’s curation and conservation director said: “Looking after such a deep and varied collection of fine art and heritage objects for visitors to enjoy requires specialist expertise and considerable funding.

“Sadly, the Trust does not have sufficient resources to address the treatment, repair and conservation of every item that urgently requires it. The Covid pandemic has presented even greater funding challenges.”

She said the National Trust’s collections needed conservation or repair for many reasons, including the effects of aging, light or damp, as well as damage from insect pests such as silverfish and moths.

The trust looks after more than one million objects at more than 200 historic properties, and many survive in their original settings rather than in bespoke museum environments, which also creates challenges, she said.

As well as preserving more than 100 works of art, textiles, books and furniture, the funding will also help the Trust to conduct technical research, acquire extra equipment and develop facilities at the studio for further expertise in painting conservation.

The Royal Oak Foundation was founded in 1973 by the National Trust, to raise awareness and advance its work by inspiring support from the United States.