One of six paintings that make up the artist's celebrated series of large-scale paintings of life on the Stour, The Lock is the last to remain in private hands. It shows a lock-keeper at the gates of Flatford Lock while a barge waits in the basin for the water-level to drop.
When exhibited at the Royal Academy, the picture sold on the opening day - a unique event in John Constable's career - for 150 guineas to the celebrated collector James Morrison (1789-1857). It was inherited by his grandson Colonel James Morrison of Basildon Park, in whose family it remained until it was sold at Sotheby's in November 1990.
There it was acquired by Sotheby's advisory board member Baron Hans Thyssen-Bornemisza for £9.8m - then a world record price at auction for any British work of art (although just shy of the £10m-15m estimate).
In a controversial move, the painting, which carries an estimate of £20m-25m, will be sold to "safeguard the future of the private collection and allow the loan of other paintings to the museum to be secured for the foreseeable future".