Rob Darvell, 45, a graphic designer and married father-of-two from East London was told the good news live on BBC Breakfast by TV art and antiques expert Curtis Dowling.
Curtis, promoting the new series Treasure Detectives, said the postcard-size rural scene is probably a view of Constable's home county Suffolk: "It's a Constable and it's never been seen properly before, so it's part of history, will cause a stir and become a celebrity item. A large section of the art world will be salivating to get their hands on it," he said.
Speaking after learning how much the picture is worth, Rob said: "In the minutes leading up to the valuation, I was so nervous that my heart was beating like crazy. Now I'm in a state of shock. This news is everything I was hoping for, and more."
Rob's father Robin Darvell, a retired consultant ophthalmic surgeon, bought the picture at an auction in Canterbury 13 years ago. "It was in a cardboard box with other bits and pieces (including a lamp and a couple of postcards) no-one else seemed to want, sold at the end of the auction as a job lot," said Rob. "Dad viewed the box before it came up for sale and spotted a faint signature, J Constable, on the back of the frame. He kept it for many years in a drawer by his fireplace at his home in Ash, Kent, but he never hung it and hardly told anybody about it. I believe my stepmum thought it was rubbish and said 'Don't be stupid… of course it's not a Constable… get rid of it!'."
Last October, Rob's father gave him the picture, suggesting he research it.
"I kept it, in bubble wrap, in a drawer in my house. Initially I did some research myself, but after a while I contacted Curtis on the internet. And of course now I'm very glad I did."
Curtis said: "I spent the best part of ten months examining this painting and its background, consulting over 30 experts and many more people besides. The picture passed every single test we put it through, including the paint which is consistent with the period and almost identical to the paint on a known Constable."
Curtis believes it was painted for and owned by Constable's father-in-law and then found its way to various relatives in Suffolk before it moved down to Kent. It probably depicts somewhere near East Bergholt, he said.
Curtis is now looking after the painting until Rob and his family decide what to do with it. "I'm much more nervous of handling the picture now," said Rob. "The frame is falling apart a bit and it's all very delicate.
"It's moving to think such a great artist painted it. I suspect we will want it hung in a gallery for a while, so the public can see it, so we don't plan on selling it for the time being at least. But, if it does get sold, the money will get split appropriately among the family."
