The three-panelled artwork was removed from the chapel during refurbishment and appraised by medieval Flemish art experts.
The charity hopes to raise between £2.5 and £3.5 million from the sale.
Richard Hunt, chair of the almshouse trustees, said: “For nearly 600 years, our mission has been to provide accommodation for local people in need and now, with the sale of this artwork, we have an important opportunity to protect the work that the charity can do for generations to come.”
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He explained that insurance, security, and climate control costs make it impractical to keep the painting on site.
The trustees unanimously decided to sell the artwork.
Mr Hunt added: “The Almshouse has been the privileged custodian of this artwork for a long time, but in light of its newly-revealed importance and value, we feel we must secure its preservation in an environment where it will be better conserved, and to use the funds to do what the Almshouse is here for, providing accommodation for local people in need in the heart of Sherborne.”
Proceeds from the sale will fund the creation of six new homes within the almshouse building, providing independent living for those facing hardship.
There are also significant on-going maintenance costs associated with keeping our Grade I listed building.
The painting will be part of Sotheby’s Sale of Old Master and 19th Century Paintings in London on Wednesday, December 3.
Mr Hunt said: “We have listened to the community around us who want the charity and the Almhouse to continue with its essential work in Sherborne.”
The almshouse encourages those interested to visit its website for more details.
