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And he has called for the auction industry to modernise its practices in the management of clients funds.

In an interview on BBC Radio Four’s Today programme earlier in the week, he had already questioned the whole structure of the guarantee system and the manner in which Sotheby’s and Christie’s have started to act like merchant banks.

Bonhams, who do not run a contemporary art department and do not hold sales as part of November’s contemporary, Impressionist and modern art series, are also critical of Sotheby’s and Christie’s for not separating client funds from their own – although Mr Brooks agreed in his radio interview that those funds were not at risk as a result.

Now he has taken the matter of guarantees further by issuing a release highlighting the risks of the system after Sotheby’s poorly performing Impressionist and Modern sale left them with a multi-million dollar bill in guarantees and saw their share price fall by a third.

Despite the later boost of the contemporary art sales, Mr Brooks believes guarantees damage auctioneers’ credibility: “The recent art sales in New York demonstrate how the widespread use of guarantees in the auction industry can be very risky,” he said.

“The minute an auction house moves away from simply being an intermediary between buyers and sellers and takes on the role of financier, it starts to change its core character as well as the relationship between the seller, his agent and the buyer.

“Consistency and clarity in core practices are essential ingredients needed to develop the trust of clients and customers alike. At Bonhams we see ourselves as agents – nothing else.

“We strongly believe that auction house clients prefer service based on the principle of agency combined of course with the complementary skills of world-class knowledge within each relative category and market sensitivity.

“The trend of an ever-rising use of guarantees among some auction houses must ultimately erode the credibility of the industry and thus its stability.

“Particularly in light of the risks being taken with guarantees, we also find it bewildering that many of our competitors fail properly to segregate their clients’ money from their business accounts, something we regard as a critical core value in our business. Just about every other area of the modern professional world has maintained such segregation for years.

“So far as the auction world is concerned, within this crucial area it now needs to put its practices in order. Change is long overdue.”