Renowned Suffolk painter Thomas Gainsborough's earliest known self-portrait is to be sold at auction.

It is believed the famous artist created the painting when he was aged 13 in 1740, soon after he had moved from London to Sudbury.

He would go on to be one of the best-known British artists of all time, painting portraits of kings and queens - as well as being a founding member of the Royal Academy.

The 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.

It has previously been exhibited at Tate Britain, The National Gallery of Art in Washington DC and the Museum of Fine Arts in Boston.

Nicolas Martineau, an associate in the fine art team at Cheffins, said: "This painting represents an exciting glimpse into Gainsborough's early career and is the first of a series of self-portraits made throughout his life, as well as being thought by scholars to be one of his earliest attempts at working in oil.

"The picture shows an important step in Gainsborough's development to becoming the leading portraitist in 18th-century England and it is a privilege to be offering it here at Cheffins in his native East Anglia."

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.