I wooed Bacon with Claridge's champagne but London gallery cheated me, says dealer

To most hungry artists, the offer would have been too good to refuse. Even to a wealthy Francis Bacon, sipping champagne at Claridge's, it seems to have been the answer to his prayers: a minimum of £50,000 per painting and a move to the books of the New York gallery that handled Picasso.

The offer was made in March 1978 by Arnold Glimcher, the influential Pace Gallery owner, at a time when Bacon, arguably the greatest British-based painter of the last century, is thought to have wanted to break from Marlborough Fine Art in London, the gallery that had pushed his work for the previous 10 years.

But Bacon did not go. Instead, he stayed with Marlborough until his death in 1992, a decision that baffled those close to him. Why he did not leave has remained a mystery. However, according to dramatic claims in what could become the most sensational legal spat the British art world has seen, the reason was simple. He was a victim of blackmail.

That is the allegation to be made in a High Court battle in February which, if proved, could make Bacon's estate up to £100m richer. On one side is Professor Brian Clarke, a friend of Bacon and the executor of his will. The professor claims Marlborough's then director, Frank Lloyd, asserted undue influence over Bacon, cheating him of millions of pounds and failing to account for up to 33 of his paintings.

On the other is Marlborough, the distinguished art house that claims it made Bacon famous and wealthy and dealt with his every whim with scrupulous fairness. Neither side has given ground in preliminary hearings since Bacon's estate launched a civil action against the gallery last year. But what no one expected was that a row over money and paintings would turn up allegations of blackmail.

The Independent reported three weeks ago that threats against Bacon had been alleged, but the full details of the allegations have only become clear since the judge, Mr Justice Patten, ruled that a statement by Mr Glimcher could form part of Professor Clarke's argument.

In it, Mr Glimcher alleges that Bacon was blackmailed by Mr Lloyd into staying with gallery.

According to High Court documents, Mr Glimcher said he had two meetings with the artist in London. "Bacon and I seemed to have an immediate rapport," he said. "By the end of the second meeting [also at Claridge's], we had reached an agreement on which we shook hands."

Bacon, Mr Glimcher said, was delighted with his promise of £50,000 a painting. But, suddenly, the artist pulled out.

Later, Mr Glimcher claims he was told by Michael Peppiatt, the respected art historian and author of Francis Bacon: Anatomy of an Enigma, about the allegations of blackmail.

Mr Glimcher said: "When Francis Bacon informed Frank Lloyd he was leaving Marlborough for Pace, Frank Lloyd told Francis Bacon that if he left Marlborough, Bacon would have problems accessing funds that Marlborough [had] paid to Bacon in Switzerland. I recall something that Bacon's sister was in a sanatorium of some kind. I was also told that there were threats by Frank Lloyd of income tax exposure."

Bacon, who had bank accounts at Dreyfus Soehne and Rothschilds in Zurich, was deeply in debt to the Inland Revenue. According to Professor Clarke, exposure would have left Bacon financially unable to care for his sister, Ianthe Knott, who was suffering from a degenerative disease in Zimbabwe, so he decided to stay with Marlborough.

In his statement, Mr Glimcher, who has been advised by his lawyers not to comment on the case, says he believes it was Mr Peppiatt who told him about the blackmail threat. Lawyers for Marlborough do not want Mr Peppiatt questioned until the full hearing in February. In another statement, however, Professor Clarke says that during a meeting in 1999, Mr Peppiatt said to him: "I suppose you will be wanting to know about the famous 'blackmail' conversation with Glimcher."

Mr Peppiatt has also been advised not to comment. It is understood he has expressed a willingness to co-operate fully with the court, but lawyers for Marlborough are unhappy that Bacon's estate has asked him to help compile a prestigious catalogue of the artist's work.

Marlborough's legal team is also concerned about the independence of Mr Glimcher. During a preliminary hearing several weeks ago, Michael Lyndon-Stanford QC, for Marlborough, pointed out that Mr Glimcher was a competitor of the Marlborough in New York. And he asked Mr Justice Patten to bear in mind that Mr Glimcher had acted for the estate of Mark Rothko when that artist had had a similar dispute with Marlborough in the 1970s.

Marlborough rejects the allegations. It said: "It remains Marlborough UK's case that a provisional arrangement was made between the Pace Gallery and Bacon (as evidenced by [a] letter of 4 March 1978).

"Bacon responded to that letter on 8 March 1978 stating that he had not made up his mind about whether to move to Pace Gallery and wrote again on 17 March 1978 stating that for the present time he had decided not to change his gallery in New York. Marlborough UK does not know why Bacon changed his mind, but would invite the court to infer that Bacon decided that it was in his best interests to continue to work with Marlborough."

That is something Bacon's estate disputes. It claims that dozens of his paintings may be unaccounted for and that he was paid only $40,000 (£28,000) for one series of lithographs when, in fact, many more than that were produced.

If the court case is successful, the recipient of any award would by Bacon's sole beneficiary, John Edwards, with whom he developed a filial relationship after the pair met in London in 1974. Professor Clarke says the primary purpose of the litigation is not to enrich Mr Edwards, but to establish a record of Bacon's work and to provide funds for research.

The judge is keeping an open mind as to the veracity of the allegations. "The court is only concerned at this time to filter out hopeless claims ... the blackmail claim does not fall into that category," Mr Justice Patten said in his judgment. "But that does not mean that it will succeed or that I have formed any view at all as to its truth."

Mr Lloyd cannot defend himself against the allegations. He died in 1998.