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But the defence is expected to challenge Mr Davidge’s evidence on at least two points: he was never in the room when Mr Taubman is supposed to have discussed the matter with ex-Christie’s chairman Sir Anthony Tennant, and Mr Davidge has admitted he lied to the Office of Fair Trading over the collusion.

Mr Davidge told the court last Wednesday that he had voiced concern about meetings between Taubman and Tennant between 1993 and 1996 in which the two allegedly agreed to fix commissions, stop guaranteeing private collections and end contributions to charities of potential clients. Sir Anthony had dismissed his warnings, he said, and briefed him to negotiate the deal further with Sotheby’s chief executive DeDe Brooks, who has already pleaded guilty to collusion charges and is expected to give evidence soon prior to sentencing in her own case.

Central to the government case is a three-page handwritten memo purportedly detailing terms of co-operation between Taubman and Tennant, which Mr Davidge says Sir Anthony handed him after a breakfast meeting between the two chairmen in 1993.

Shortly afterwards, says Mr Davidge, he was told by Tennant to expect a call from Mrs Brooks with a view to applying the pact. Over the next 18 months, the two executives agreed to set identical higher-end charges for vendors.

Mr Taubman, in the dock, and Sir Anthony, who has declined to travel to the United States to face charges, deny any wrongdoing.