The Royal Society is trying to block the auction of a 17th-century manuscript that charts the beginning of modern science.
Britain's premier scientific academy believes the 520-page document, by the scientist Robert Hooke, may have been stolen from its archives 300 years ago.
But the decision to try to block the sale, due on Tuesday, has infuriated the auctioneer Bonhams, which has accused the society of duplicity.
"They are negotiating on one hand to buy and seeking public funding for this while publicly issuing threats or bringing pressure to bear via the media," said Julian Rupe, a spokesman for Bonhams.
"All we can conclude is that perhaps they worry the manuscript will sell for a higher price than they can afford. This is a rather unacceptable negotiating tactic."
He said Bonhams had told the Royal Society of the discovery soon after valuers came across the document at a home in Hampshire in December, and had allowed it to be viewed at the Royal Society.
Bonhams also introduced the society to potential benefactors who might buy the manuscript on its behalf. "We have bent over backwards to keep them in the loop," Mr Rupe said. "You can imagine that we were rather surprised subsequently to read in the press that they had sought legal advice about the sale."
The document, valued at £1m, consists of Hooke's minutes of Royal Society meetings between 1661 and 1682. Hooke was one of the society's original fellows and he peppered his notes of the meetings with acerbic asides. His notes were later transcribed as official minutes, which the Royal Society still holds.
The Royal Society is keen that the manuscript does not go to a foreign bidder because of its historical significance.
"It's a great pity that the Royal Society cannot itself afford to purchase them so that they could be restored to our collection of documents from which they were removed at some point during our early history," said Lord Rees of Ludlow, the Royal Society's president, when the documents came to light.
The manuscript contains descriptions of the first glimpses of sperm and micro-organisms using an early microscope and discussion between Hooke, Sir Christopher Wren and Sir Isaac Newton over the nature of gravity. Mr Rupe said the society's position was "inconsistent" because similar documents were owned by other institutions and individuals. Manuscripts by former Royal Society president Sir Hans Sloane were owned by the British Museum, for example. The Royal Society declined to comment.