Discovered: the 500 missing masterpieces of J M W Turner

A global search by the Tate gallery has unearthed 500 lost works by JMW Turner. Some have never been seen in public; others have not been displayed since the death of possibly Britain's most important painter.

The priceless haul, considered one of the most significant discoveries in the British art world for decades, came about as a result of unprecedented co-operation between private collectors, small galleries and the Tate. "It has taken a year to find the missing 500. The Tate has had to start from scratch. They didn't have a clue where they were," said a spokeswoman for the London gallery last night.

The rediscovered works form part of a larger collection of more than 2,000 scattered pictures located during the Turner Worldwide project. The discoveries will bring into the public domain together for the first time some 200 oil paintings, around 1,800 watercolours and 200 pencil drawings, many of which were stored forgotten or hanging mislabelled in small galleries around the world.

The search began with a website appeal urging private owners, curators and dealers to come forward and help to locate the hundreds of missing works. The plea brought an unprecedented response.

Turner watercolours have sold at auction for as much as £2m in recent times while the artist's oil paintings are even more valuable, fetching between £5m and £10m for the finest works.

Tate experts have photographed all the new discoveries and placed them on the internet, creating the largest and most important online gallery of any artist.

Sandy Nairne, director of the National Gallery, said last night that the tracing of the missing works was "extremely significant". He said: "It's great that private owners have been so responsive, particularly when people are so worried about privacy. They see the larger issue of why Turner matters so much and why he is so important."

Guy Peppiatt, head of the British watercolour department at Sotheby's auction house, said the development was "really incredibly important". "The most important artist we sell from a value point of view is Turner. He is arguably considered the greatest British artist," he said.

The project is still attempting to locate 400 pictures that the artist is known to have produced. Details are to be posted again on the Tate website in the hope that the current owners may yet come forward.

So far Turners have been traced in museums from Hyderabad in India to Australia. One, Landscape with Woman and Tambourine, was supposed to have been sold to America but was found in a gallery outside Tokyo.

Other works were stored at the National Gallery of Ireland in Dublin and at the National Galleries of Scotland in Edinburgh but were not allowed to be moved at the insistence of the person bequesting them, for fear they might be damaged.

Some collectors were disappointed after their paintings were identified as being the work of other 19th-century artists who had imitated Turner.

Among the Turners that have been tracked down is Harlech Castle, a watercolour painted in the 1830s. The writer John Ruskin, who championed Turner's work, bought the painting for £63 in 1840, but it is now likely to be worth hundreds of thousands of pounds. The watercolour was sold in 1882 and had not been heard of until the Turner Worldwide project traced it to a private collector on the east coast of the United States.

Another work, Scene in Derbyshire, had not been seen since being exhibited at the Royal Academy in London in 1827. It was "discovered" hanging in the Musée national des beaux-arts du Quebec, where it was described as Near Northcourt, a mistaken reference to another Turner location on the Isle of Wight.

St John's Church at Margate was produced when Turner was a very young artist and was sold to a private collector. When it goes on display on the online gallery today with the other missing Turners it will be the first time the work has been publicly exhibited.

The Turner online gallery, which was set up with the help of BT and the National Lottery new opportunities fund, also includes high-resolution, thumbnail images of the 30,000 works left to the Tate by the Turner Bequest and is now the definitive catalogue of the artist's work.