Controversy as 'Velazquez' masterpiece comes to auction: Experts disagree on provenance of 'Immaculate Conception' from 1620. Geraldine Norman reports

IS IT a Velazquez or not? On Wednesday Sotheby's will offer for sale The Immaculate Conception which everyone agrees was painted in Spain in about 1620. The auctioneers say it is by Velazquez and worth about pounds 7m. They are supported by the two leading Velazquez experts in the US, Jonathan Brown and Bill Jordan. But two former directors of the Prado in Madrid, Alfonso Perez Sanchez and Francisco Calvo Serraller, think it is not.

The question whether the Virgin Mary was sanctified in her mother's womb after conception or sinlessly conceived was settled by Pope Paul V's brief of 21 August 1617 in favour of immaculate conception. Many Spanish religious institutions then commissioned paintings whose iconography was based on the apocalyptic vision described in Revelations of a 'woman clothed with the sun and with the moon under her feet and having upon her head a crown of 12 stars'.

Velazquez was commissioned to paint an immaculate conception for the convent of the Shod Carmelites and the painting is now in the London National Gallery. There are two similar paintings in Seville by Francisco Pacheco, to whom Velazquez was apprenticed from 1613 to 1618.

Perez Sanchez argues that Sotheby's painting is the work of another Pacheco apprentice, Alonso Cano. It was certainly painted in Pacheco's studio where the two artists shared brushes, he told the Spanish newspaper El Pais. So the argument turns on whether you can recognise the hand of the master in the details, such as folds of cloth or hands.

Sotheby's painting turned up in an auction at the Paris Hotel Drouot in 1990. It was catalogued as 'circle of Diego Velazquez' and estimated at pounds 30,000 to pounds 40,000. But on the day, two bidders - Charles Bailly, the Left Bank picture dealer, and a collector called Chevreux - were determined that the painting was by Velazquez. Bailly secured the painting at 18m francs ( pounds 1.8m).

Bailly believes that Chevreux had been enouraged to bid by the Louvre in the hope that his collection will eventually be donated to the nation. Pierre Rosenberg, the chief curator, had seen the painting before the sale and French law would have permitted him to pre-empt the purchase of the painting on behalf of the state at the auction price.

'He was probably worried the owners would demand he pay them the difference between the auction price and the price of a real Velazquez,' Bailly says. The Louvre was recently sued for buying a bargain at auction and had to pay up.

Bailly has covered himself by paying the owners more than pounds 100,000 compensation in return for their dropping any claims on the picture. The painting is said to have belonged to two old sisters in Paris who had it hanging in a maid's room.

Bailly is famous for his discoveries; he has recently been denied permission to export two newly discovered paintings from France, a Rubens and a Poussin. It took the Musees de France six months to decide whether to let the 'Velazquez' out and, according to Bailly, permission only came through after his lawyer pointed out that his client would go bankrupt if none of the three paintings were allowed to leave the country. The Louvre, which owns no genuine Velazquez, had the right to buy it at the export valuation of pounds 14m, but again turned down the opportunity.

Before deciding to include the painting in a public auction, Sotheby's shipped it on approval to the Getty Museum in California. Experts at the richest museum in the world were worried about the condition of the painting and turned it down, according to Sotheby's.

In agreement with Bailly, they then decided it should be cleaned before being offered for sale. It was after cleaning that American scholars became convinced that the brushwork was that of the master.