Looking good: Mr Faryab has had the painting cleaned and reframed and has gathered scientific evidence, checking fingerprints and artistic tests to prove its provenance

When Frank Faryab bought an obscure oil painting for thousands of pounds in a private sale, it was just the start of his outlay on the work.

For the art and antiques dealer has since spent more than £2million and much of the past five years trying to convince others it was by JMW Turner, one of Britain's greatest painters.

The perseverance appears to have paid off - because three leading authorities on the artist work now agree it is a lost Turner masterpiece which could be worth about £4million.

Historic: The picture, painted in October 1844, depicts the boat of the French king, Louis-Philippe, arriving in Portsmouth Harbour

Experts at the auction house Sotheby's will examine the painting in detail next month and may accept it for sale.

The highest price ever fetched by a Turner work is £29.7million.

Mr Faryab, who lives in Cirencester, Gloucestershire, was tipped off in 2007 that the painting might be by Turner, although it was sold as a minor work by another artist 'in the manner of Turner'.

He will not say how much he paid for the seascape, a 20in x 16in oil-on-pine panel of a hazy sailing ship, but he can list the great lengths he went to for the painting to gain recognition.

He has had the painting cleaned and reframed and has gathered scientific evidence, including infrared dating, checking fingerprints and artistic tests to prove its provenance.

Mr Faryab told the Sunday Times: 'Years of neglect, dirt, soot and nicotine had left their mark

'But I insinctively knew its true quality, which was why I was prepared to invest so much on research proving my judgment.'

Three Turner experts have been to Geneva, where the painting is kept, and all have reported either categorically or 'almost certainly' that the work is by Turner.

One of them, James Hamilton, of Birmingham University, described the work as a 'magnificent Turner'.

He believes that the picture was kept in the artist's studio until his death in 1851, and then sold on by John Pound, the son by a former marriage of the artist's last mistress, Sophia Caroline Booth.

'It just struck me as absolutely right,' the academic said. 'His brush strokes are all over the work.

'It is as if you can feel Turner coming out of the painting.'

James Miller, former deputy chairman of Sotheby's, and the London art dealer Lowell Libson also believe Mr Faryab has bought a Turner.

The picture, painted in October 1844, depicts the boat of the French king, Louis-Philippe, arriving in Portsmouth Harbour.

The French king's journey is well recorded in the papers of the day and in five other paintings owned by the Tate - all Turners.

Another fact which supports Mr Faryab's claim is that Turner and Louis-Philippe were friends.

The king even gave the artist a snuff box.

When Mr Faryab bought the painting, it was called Ships at Sea. It now has the grander title The Arrival of King Louis-Philippe.

Turner produced several works a day - mostly drawings, etchings and watercolours - and authorities believe it could be one of about 540 oils the artist is thought to have painted.

Four years before Mr Faryab bought the painting, the Tate identified two of its Turner works, originally thought to be of Venice, as the French king's arrival.

Mr Faryab's painting looks similar to a scene from those pictures, and to another Turner at the Tate called Hurrah! For The Whaler Erebus! Another Fish!

Newly uncovered oil paintings by Turner are rare. The last significant one, Landscape with a Rainbow, was identified in 1995.

The record price for a Turner work was set in 2010, when Modern Rome - Campo Vaccino was sold for £29.7million.