Dossier sheds light on the Tate's £700,000 payment to Chris Ofili

Stuckism International, a group of artists critical of the conceptual work it claims the Tate favours, has submitted its own dossier about the affair to the Charity Commission. Charity commissioners have already asked the gallery to explain itself.

Questions were raised after documents obtained under the Freedom of Information Act detailed exchanges between Nicholas Serota, director of Tate galleries, and Victoria Miro, Ofili's dealer, in which she emphasised that the artist was getting married and was unlikely to want to wait long to be paid.

The papers revealed that the Tate paid £600,000 for The Upper Room, a series of 13 paintings. The £600,000 price is without VAT, which brought it to £705,000.

The Tate and its members supplied £220,000, with the rest coming from benefactors. The Charity Commission has written to the Tate asking it to address issues raised by the row including the question of conflict of interest, although a spokeswoman stressed this was not - yet - an official investigation.

Charles Thomson, co-founder of the Stuckist movement, said in his submission to the commission that there were other questions that needed to be answered.

The Museums Association guidance on acquisitions states that the value of any potential purchase should be researched and at least one independent valuation sought. "There is no indication in the trustees' minutes that there has been any research to establish the value of the item," Mr Thomson said.

He also argued that there was a conflict of interest for Sir Nicholas Serota, who was appointed to his post by the trustees. "As his employment is in their remit, it creates a conflict of interest when he is involved in procedures which directly affect a trustee's interest," he claimed.

And he said the role of the private benefactors who contributed to the acquisition should be examined. All were private purchasers of Ofili's works from the Victoria Miro Gallery in London, who therefore stood to benefit from the boost to the artist's status and prices that would stem from the Tate acquisition, Mr Thomson said.

Two months before the announcement of the purchase of The Upper Room, Charles Saatchi, the art collector, sold another Ofili work at auction for more than £500,000, four times the previous highest price for an Ofili. "A leading question is whether the impending announcement by the Tate was known by others involved in the auction," Mr Thomson said.

However, a Tate spokeswoman defended the purchase. "Chris Ofili's The Upper Room was acquired because it is an exceptional work, marking a major development in Ofili's career.

"The Tate trustees felt strongly that to neglect to acquire this major group of paintings would represent a missed opportunity and it was acquired with support from the Art Fund, Tate Members and a group of individuals."

It was the gallery's policy only to purchase work by serving artists in exceptional circumstances, she said. Such cases were debated in full, in the absence of the artist in question.