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“He knew instinctively that the painting was extremely valuable,” said specialist John Harwich, although the buyer may well have got the painting for a lower price had he not been chased by another dealer. Mainstreet Auctions were unavailable for comment, but it is believed that the painting had come to light during the routine clearance of a local house.

Mr Harwich, head of British and Irish Art at Christie’s, was delighted to have received such an important painting, and only just before the March 24 deadline for entries to the Irish Art sale had expired. Unlined and in absolutely untouched condition with a subtle colour beneath a thin film of grime, the oil is expected to be one of the highlights of the sale and will carry an estimate of £100,000-150,000.

Similarly sized oils by Leech have been known to perform above this level of value. At Christie’s sale of Irish Art on May 22 1998, The Blue Shop, Quimper, a 24 by 16in (61 x 41cm) oil on canvas by the artist, sold for what is believed to be a record price of £265,500.