Under a Kirghiz Tent, by Russian artist Alexander Yakovlev.

It was supposed to be the ultimate Antiques Roadshow moment. Several months ago, officials at the Attleboro Arts Museum made a startling discovery: a painting that had been kept in storage for decades was actually worth as much as a million dollars. The discovery prompted the museum to put the painting — Under a Kirghiz Tent by the mid-20th-century Russian painter Alexander Yakovlev — up for auction, where it was expected to bring from $800,000 to $1 million.

Yesterday, those expectations were dashed when the painting failed to sell at a major auction of Russian art in London. The outcome is a blow to the Attleboro Arts Museum, a small city-owned museum that had hoped to use the money from the sale to beef up its endowment and expand its community outreach programs. It also leaves the fate of another Yakovlev painting — this one owned by Attleboro’s School Department — in limbo.

Both paintings were donated to the city in the 1950s by Charles Thompson, a longtime Attleboro resident who worked as a salesman for the Vose Galleries, a Boston art gallery that also represented Yakovlev. Though both works were discovered about the same time, the Attleboro Arts Museum painting was the first to come on the market. City officials are still debating what to do with the School Department painting, which depicts a group of Afghan tribesmen and is estimated to be worth between $600,000 and $800,000.

bvansicl@projo.com