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Ms Brooks, a former Sotheby’s chief-executive who has already pleaded guilty to price fixing charges and whose sentence depends on her testifying, said that while he was Sotheby’s chairman, Mr Taubman had directed her to meet her counterpart at Christie’s, Christopher Davidge, to raise prices charged to vendors and fix them at identical rates. He even wanted to agree with Christie’s on fixed estimates for works of art, she said, but this was rejected because specialists were unlikely to comply.

Mr Taubman congratulated her for carrying out his orders, she told the court, but when the conspiracy was uncovered by federal investigators in January 2000, he warned her not to lose her nerve.

Mr Taubman has pleaded not guilty to all charges of violating US anti-trust laws, maintaining that Ms Brooks and Mr Davidge conspired without his knowledge. Both individuals have admitted repeatedly lying to cover up their role in the price-fixing scheme – Mr Davidge after testifying that he was acting under orders from his then boss at Christie’s, Sir Anthony Tennant, who has been indicted but cannot be extradited from Britain.

The defence sought to undermine Ms Brooks’ credibility, saying that the potential value of her stock options in Sotheby’s gave her a personal motive to conspire with Christie’s to eradicate competition, but this argument fell flat when the prosecutors argued that Mr Taubman had far greater holdings of over 13 million shares. However, the defence successfully highlighted Ms Brooks’ deceit by contrasting her public statements to the press with her illegal dealings behind the scenes with Christie’s. When Sotheby’s faced accusations in 1997 of smuggling Old Masters out of Italy, Ms Brooks told a journalist she took questions of ethics and integrity “very seriously”, adding that her name was her sole asset. In an interview over the smuggling, she again held up the auction house’s integrity as an unassailable asset used to set new standards.

Mr Taubman’s defence also accused Christopher Davidge of selling his testimony to the highest bidder. Under cross-examination, Mr Davidge admitted that he only agreed to testify after Christie’s promised him $8m in severance pay, $3m of which is still to come. Christie’s needed his testimony to seal the amnesty deal struck with prosecutors.

Whether or not Ms Brooks will escape a three-year prison term remains to be seen. She will be sentenced after the trial. But Mr Taubman’s fate may now depend on his willingness to take the witness stand in order to defend his besmirched reputation.