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But, in making her plea, she claimed she had only been acting under orders from her former boss, ex-Sotheby's chairman Alfred Taubman - a charge he strenuously denies.

Admitting one felony charge, Ms Brooks read out the following pre-prepared statement in court: “At the direction of a superior, I had a number of meetings and conversations with a representative of Christie’s International in which, among other things, I agreed to fix prices with respect to the commissions charged to sellers.”

Sotheby’s also pleaded guilty to criminal charges of price-fixing and other anti-trust violations, agreeing to pay a $45m fine over a five-year period. However, the presiding judge in the criminal case, Lewis Kaplan, has suspended his approval of this settlement until an end-of-year hearing into Sotheby’s financial capacity to pay litigants in the class-action case the sum of $256m agreed two weeks ago.

“The behaviour that led to today’s plea was wrong and is unacceptable,” said Sotheby’s new chief executive, William Ruprecht, after the court hearings.

“I apologise to our clients for this breach of the standards of trust that they have the right to expect from us and assure them that no member of Sotheby’s current management played any role whatsoever in these events or was aware at any time that they were taking place.”

Assistant attorney general Douglas Melamed went straight to the point: “Those charged today were engaged in classic cartel behaviour – price-fixing pure and simple.”

Sotheby’s could recoup their $300m losses ($156m of which Mr Taubman has agreed to pay in return for avoiding any legal action from the company) if they can prove to their insurers that they cannot be held responsible for criminal actions undertaken by individual employees.

Brooks, who resigned with Taubman when the collusion scandal broke in February, now faces the possibility of three years in prison and a fine of more than $100m when she is sentenced in January next year, but her lawyers are seeking leniency in return for their client’s co-operation, possibly as a star prosecution witness, in the ongoing criminal investigation into price-fixing allegations which will now focus on Mr Taubman.

According to a report in the New York Times, documents in the hands of Federal investigators show that Mr Taubman and Sir Anthony Tennant (former chairman of Christie’s) instructed their respective chief executives, Diana Brooks and Christopher Davidge, to carry out the scheme as early as 1992.

Christie’s managed to secure amnesty from Federal criminal charges by handing over documents left by Davidge which pertained to the anti-trust investigation. However, Christie’s has been threatened with “sanctions” by Judge Kaplan in a case brought against both auction houses by overseas litigants (who are not covered under the $512m civil case settlement for buyers and sellers in America) unless Christie’s clarifies references to unnamed people in the Davidge document.