'Lost' Constable watercolour hints at glory to come in 'The Hay Wain'

The inclusion of a hay-cart in the centre of this watercolour anticipates the core motif of the most famous of John Constable's mature landscapes, The Hay Wain, 15 years later. But no one even knew this finished watercolour by the British artist existed, until it came to light in the United States earlier this year.

Subsequently acquired by the Tate, it will be seen in public for the first time this week as an unexpected addition to a long-planned exhibition dedicated to the great Constable landscapes.

What has excited scholars about the discovery of this work, entitled The View in the Stour Valley Looking Towards Langham Church from Dedham, 1805, is that only one finished, major watercolour painting from Constable's early years was previously known. The newly discovered painting fills in a gap in the understanding of Constable's development. He was born in Suffolk in 1776 and enrolled at the Royal Academy Schools in 1800, but his early career is comparatively poorly documented.

The find shows he was keen on competing with the metropolitan elite, who had all turned to watercolour as the medium of the moment. It also suggests the seriousness of his ambitions as a landscape painter, even from these early days.

Anne Lyles, the curator of the new Tate Britain exhibition, said: "It shows his readiness to experiment and take on the new medium, but on a big scale, which is fairly new for that date."

Before 1805, Constable had only occasionally used watercolour and when he had, it was for rather conventional late 18th century "tinted" drawing. But this is a full exhibition-scale work comparable only to the finished painting His Majesty's Ship Victory, commemorating the Battle of Trafalgar, now in the Victoria and Albert Museum.

The View in the Stour Valley appears never to have been displayed, and was conceivably painted as a gift for Bishop Fisher, an early supporter and mentor. It is understood to have been in the same family collection in America for years, although there are no clues to how it got there. A knowledgeable New York dealer recognised its importance and contacted the Tate. "There's some missing stuff we know is missing, but this is one we didn't even know was missing. [But] it doesn't necessarily mean there are more," Ms Lyles said.

The exhibition will bring together for the first time Constable's seminal six-foot canvases including The Hay Wain and Salisbury Cathedral From the Meadows. They were never seen together even in his lifetime.

Constable: the Great Landscapes, sponsored by AIG, opens on Thursday and runs until 28 August. Admission is £10.