A council which is selling an LS Lowry painting to balance its books says it will be auctioned in two months.
The Riverbank was bought in 1951 by Bury Council in Greater Manchester for £175, but could now fetch £500,000.
The Museums Association has said the decision to sell the painting to plug a £10m deficit is "as bad as it gets".
The director of the professional body told BBC News that the sale was inconceivable and that it still hoped to dissuade the council from selling.
Mark Taylor said the MA had written to Bury Council, urging it not to go ahead with the auction, on 17 November at Christies.
He said the sale was a breach of trust and unethical and would lead to it being disciplined.
The council's accreditation to the government's Museums, Libraries and Archives Council (MLA) is also under threat.
Its loss would mean Bury Museum and Art Gallery, where the Riverbank is displayed, would not be eligible for public money, or lottery funding.
The MLA, which represents the vast majority of public art galleries and museums in the UK, has said that auctioning a painting to benefit council finances was against its guidelines.
The council said it had considered selling The Riverbank at a lower price with a "leaseback" option, allowing it to remain on display in Bury, but no buyer was found.
Wayne Campbell, leader of Bury Council, said: "We are devastated that we have been forced to realise funds from our art collection in order to meet the budgetary shortfall, but the alternatives such as redundancies and closures of valued services were equally as unpalatable.
"We are committed to reinvesting the money into cultural projects... and to saving services to better meet the demands of the local community."
LS Lowry is most famous for his matchstick men and industrial landscapes, based largely on Manchester and Salford.