While Hampstead’s John Constable is revered as one of Britain’s greatest artists, a new discovery has revealed that another painter and decorator often had a helping hand in some of his masterpieces.

At the height of his fame Constable – a self-confessed slow worker – was inundated with commissions. This week it has been revealed that the great landscape painter cut corners to meet deadlines.

A long-lost 1824 painting, unearthed after 62 years, has given a fascinating new insight into his methods and will be auctioned at Bonhams.

A painting of horse-drawn wagons on Hampstead Heath has shown that Constable’s studio assistant Johnny Dunthorne would sketch “rather crude” outlines of figures – based on Constable’s own paintings – on vast canvases to be finished by the master.

David Dallas, international director of old master paintings at Bonhams, said: “It does give a very interesting insight into the way Constable worked. “He had all of these commissions which had to be done in a rush and he admitted himself he was a bit slow, but this allowed him to get those commissions out on time.”

The revelation comes after the painter’s depiction of horse-drawn wagons was unearthed in the hands of a private collector, who lives overseas.

The work has been valued at between �60,000 and �80,000 and will be sold on December 5 at Bonhams in New Bond Street.

The outline is an identical match to the figures in the foreground of Constable’s seminal work Branch Hill Pond that was produced in 1824 at the height of his fame.

Mr Dallas made the discovery when he was inspecting the figures in the painting.

He found incisions in the work where Dunthorne, a house painter by trade, would have traced over the painting while it was still wet.

The art expert said: “I don’t think it diminishes Constable’s legacy at all really. He confessed he had trouble introducing figures into paintings.

“He worked up a nice composition and thought that was useable and this is how he got Dunthorne to incorporate it.”