A ancient Chinese scroll, which went under the hammer for $8.2 million at an art auction in New York, has become the focus of an intense debate between the auction house that sold it, the wealthy collector who bought it, and a trio of museum experts who have denounced it as a fake.

The public spat began Saturday when three art experts from the Shanghai Museum alleged that the calligraphy work, titled "Gong Fu Tie," was a forgery. Purchased at a Sotheby's auction in September by collector Liu Yiqian, the piece is attributed to the Song Dynasty poet Su Shi, who lived from 1037 to 1101 and is known by the literary name Su Dongpo.

The three specialists scrutinized the work in the Xinmin Evening News, a state-owned newspaper based in Shanghai. They added that they plan to release a research paper giving more details about why they believe the piece to be a counterfeit.

In response, Sotheby's released a statement saying that it "firmly stands by" the work, which it hails as a masterpiece of its kind. A long hanging scroll with nine ink characters, the piece is a farewell letter from Su Shi to the historical figure Guo Xiangzheng, who went by the literary name Gong Fu.

The allegations have left Mr. Liu, the scroll's current owner, embittered.