LONDON (Reuters) - A small painting bought at an auction and kept in a cupboard for years, has been revealed as an original by British painter John Constable worth an estimated 250,000 pounds ($400,000).

The postcard-size painting, bought for 30 pounds in the British city of Canterbury around a decade ago, depicts a landscape by the 19th century artist and has been described as a "lost item" by the man who uncovered its origins.

Antiques dealer and forgeries expert Curtis Dowling told Reuters he and his team spent about nine months studying the painting after the owner, Rob Darvell, asked for their help.

Darvell's father, who bought the painting suspecting it may be an original because of the faint signature on the back, had given him the painting as he was clearing his house.

"It's a fairly standard stock sort of Constable painting. It's quite interesting in that it's small, which you don't see that often. It's something we've never seen before ... It's really actually been quite a lost item," Dowling said.

"Our investigation confirms this thing has passed through a number of hands over the years and it's never been sold - it's a fresh-to-the-market, sweet little item."

Constable, born in Suffolk, is famous for his landscape paintings of Dedham Vale, now known as Constable Country. His most famous work "The Hay Wain", painted in 1821, now features on millions of prints hanging on walls around the world.

In May, the Tate Britain museum bought Constable's "Salisbury Cathedral from the Meadows" for 23.1 million pounds, making it one of the most expensive British paintings ever sold.

Dowling, who has appeared on numerous television shows - his latest "Treasure Detectives" looks into the authenticity of all sorts of goods, described the painting as small and "not in the best condition" depicting trees, fields, horses and a man.

He said his team conducted technical tests, researched its past and spoke to art experts, some of whom may not have always wanted to commit, to dig up its origins.

The painting was originally owned by Constable's father-in-law and kept in Suffolk, eventually making its way to Kent before changing hands, he said.

"We knew it was consistent with the period. The provenance ... brought us to the conclusion, with everything else added in, that what we're looking at is exactly what we thought it was," he said. "It kind of all fitted into place."

Asked if he was certain it was a Constable, Dowling said: "I think it comes to a point where no stone has been left unturned and there are no stones left to turn over."

Dowling, who told Darvell the estimated value on television, said the painting was likely to divide the art world.

"Every time an item is discovered ... there are going to be two sides - the side that accepts all the work that has been done on it and there is going to be another side to it that is going to be scathing," he said.

For now the painting, described by Dowling as a "celebrity item", is in a bank vault and will eventually go on public display before Darvell decides what to do with it.

"What's great about it is that as we speak there are many, many more sitting in England in people's cupboards, lofts and in their boxes in the garage," Dowling said.

"And these stories will keep coming up."

($1 = 0.6425 British pounds)

(Editing by Paul Casciato)