A lost portrait by legendary Suffolk artist Thomas Gainsborough could be worth more than 400 times its £2,500 sale price - with an expert believing it could actually be worth £1million.

The painting was simply entitled "British School" at a recent Paris auction and had no reference to the word-renowned painter, who was a favourite of kings and queens.

However Hugh Belsey, from Bury St Edmunds, who is an authority on Gainsborough, believes the painting dates back to 1768.

It is thought that time, alterations, overpainting and dirt all contributed to disguise the true artist's hand on the piece.

"If you have looked at Gainsborough's work for as long as I have then it's like looking at your mother's handwriting.

"The pose, the palette, the relationship between the figure and the size of the canvas all pointed to his authorship."

East Anglian Daily Times: Hugh Belsey pictured with his 2002 book on GainsboroughHugh Belsey pictured with his 2002 book on Gainsborough

London-based painting conservator Simon Gillespie restored the portrait of composer Antonin Kammel, born in Bohemia, now the Czech Republic, who worked in Britain from 1765 until his death in 1784.

East Anglian Daily Times: Mr Belsey used to be curator at Gainsborough HouseMr Belsey used to be curator at Gainsborough House (Image: Archant)

Mr Gillespie said: "It was quite easy to walk past without a second thought.

"However, there were indications that it might be by Gainsborough himself, tell-tale confident brushstrokes.

"With the current owner’s agreement, the old varnish and mismatched repaint were carefully removed, the old tear sympathetically repaired and the old abrasions sensitively reintegrated with retouching."

Thanks to this work, former curator of Sudbury museum Gainsborough House, Mr Belsey thinks "any doubt" of it not being a Gainsborough has been removed.

He also claims that Mr Kammel is the person in the portrait, as he gave concerts in 1768 and 1769 in Bath when Gainsborough was there.

The portrait dates to this period and their mutual friendship with George Pitt must have encouraged the commission.

The portrait also shows the composer with some sheets of music in his hand, a further telltale sign.

East Anglian Daily Times: Thomas Gainsborough's portrait of composer Antonin Kammel is 76.2cm x 63.5cmThomas Gainsborough's portrait of composer Antonin Kammel is 76.2cm x 63.5cm (Image: Thomas Gainsborough)

Mr Belsey, who catalogued all the known portraits by the artist in his book Thomas Gainsborough, was awarded the William W B Berger Prize for British Art History in 2020.

He added that during his time researching the book, he found "a surprising amount of his work has been altered, even shortly after they were painted".

These changes would be done by Gainsborough, who would update ladies' fashion to the new style years later when asked by those he painted.

He would not do their faces, which Mr Belsey jokes "was very much the botox of the time".

Who was Thomas Gainsborough?

Gainsborough, a weaver's son, was born in Sudbury in 1727.

He trained in London and set up in practice in Ipswich in about 1752.

In 1759 he moved to Bath, a fashionable spa town, attracting many clients for his portraits.

He settled in London in 1774. Among his portraits were King George III and Queen Charlotte.

He was said to be a favourite painter of George III and his family.

Gainsborough's earliest known self-portrait, which has been in a London-based private collection since 2008, is expected to fetch more than £40,000 when it is sold by Cheffins in Cambridge on April 21 as part of The Fine Sale.