Sir Anthony Blunt, Surveyor of the Queen's Pictures and a Russian spy, once dismissed Landscape with Bacchus and Ceres as the work of a follower of the French painter.
Now the work, painted in 1624 when Poussin was a struggling young artist working in Rome, is being hailed as a "major find" by European art experts at the Walker Art Gallery in Liverpool.
Xanthe Brooke, curator of European art at the gallery, said: "Recently a much larger painting, which Poussin painted probably in the year before this, was valued at pounds 4m."
The painting was attributed as an early Poussin after restoration work at the award-winning Conservation Centre in Liverpool where the canvas was cleaned and X-rayed.
Its authenticity was verified by Pierre Rosenberg, of the Louvre in Paris, and the National Gallery in London.Reuse content