A painting by Wales’s most influential artist of the 20th century is to go under the hammer.

The landscape of Capel Soar in Gwynedd by Sir Kyffin Williams is expected to fetch between £20,000 and £30,000 when it is sold later this month.

It is being sold alongside an illustrated letter from the artist saying he had eaten too many peppermint sweets and would not be picked to play rugby for Wales.

The letter, which was dated December 10 1980, was written to the family who purchased the painting and who are now selling it.

Sir Kyffin wrote: “Dear Ann, Your present of the most delectable sweet-meats was vastly appreciated by the greedy recipient.

“But alas it has its disadvantages since after consuming them I will certainly be unfit for selection to the Welsh team to play England at the National Stadium in January.

“My natural sylph-like figure will be in some disarray and I fear I shall be dropped. But by God it will be worth it.

“Many thanks. Very best wishes to all Kyffin.”

Sir Kyffin, who was born in Anglesey and died in 2006, was well known for his iconic landscapes, which were scraped in thick oil with his trademark pallet knife.

The oil on canvas, which measures 50cm x 111cm, is signed with initials and inscribed on the reverse.

The Capel Soar painting, which is believed to be the most remote chapel in Wales, was exhibited at the National Museum of Wales in 1987 in a Kyffin Williams Retrospective Exhibition.

It will be sold on May 13 at Parker Fine Art Auctions in Farnham, Surrey.

Henny Smith, director of Parker Fine Art Auctions, said: “This is not just a superb example of Kyffin Williams’ work in typical limited palette and thick impasto, but the lot also includes a charming illustrated letter from him.

“The illustrations show him ‘Before’ as a fit, sporty rugby player standing to attention with the ball tucked under his arm, and ‘After’ as a bedraggled pot-bellied player who has dropped the ball.”