The cleaning process revealed details long hidden under a coat of varnish applied in 1948-49 and since badly degraded.
An 18-month project to restore Leonardo da Vinci's "Virgin of the Rocks" revealed the Renaissance artist likely painted the entire work himself rather than, as previously thought, with the help of his assistants.
The painting, a later version of one on display in the Louvre in Paris, also appears to have never been fully finished, according to the conservation team at Britain's National Gallery where the work will go back on display later on Wednesday.
The cleaning process revealed details long hidden under a coat of varnish applied in 1948-49 and since badly degraded, which had reduced the picture's subtle shading, particularly in darker areas, and impacted its intended sense of space.
The conservation work and study of materials and techniques showed different parts of the painting reached different stages of completion, the gallery said, with the angel's hand barely sketched while the heads of the main figures appear completely finished.
"In the past, gallery curators, like many scholars of Renaissance painting elsewhere, have explained the different levels of finish and resolution in the picture by arguing that Leonardo was helped by assistants," the gallery said in a statement.
"It now seems possible that Leonardo painted all the picture himself, leaving some parts just sketched or yet to be completely resolved, and others fully worked up."
In 2005, experts using infrared technology discovered two distinct underdrawings beneath the surface of the painting, which was bought by the National Gallery in 1880.
One was a completely different design from that eventually painted over the top of it, while the second is a drawing of the Virgin of the Rocks but with considerable changes.
The restoration project, which was carried out by a team which combined curators with the gallery's conservation and scientific departments, also involved discussions with experts in other countries, including at the Louvre.
The painting, dating from about 1491 to 1508, will be displayed in a new frame which incorporates parts of a late fifteenth century Italian frame bought specially by the gallery last year.
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