Although Britain is no longer the global force it was in industries such as steel production or ship building, it is still a big fish in the relatively small pond of the international art market. In 2017, the country was the third-largest art trading hub, with total sales of $12.9 billion, according to a report published last March by Art Basel and UBS. The United States had the largest art market, at $26.6 billion, followed by China, with $13.2 billion. The figure for Britain represented 62 percent of sales in the European Union, with France the next biggest in the bloc, at 22 percent.

In a recent article, the author of that report, Clare McAndrew, wrote that Brexit was a “golden opportunity” for the art market in Britain. At 5 percent, its import tax on artworks is the lowest in the European Union. If, after Brexit, Britain were to reduce or remove this tax, it could “attract global sales, effectively bypassing Europe altogether,” Ms. McAndrew said.

That prospect alarms art dealers on the Continent. This month, the French trade federation for the art market wrote an open letter to the French government calling for changes to counter the competitive threat of a deregulated British art trade. “What we fear is a liberalization of the U.K. market, with relaxation of the financial burdens and constraints,” Mathias Ary Jan, president of the Syndicat National des Antiquaires, one of 10 association heads who signed the letter, said in an email.

With buyers in the European Union currently accounting for less than 20 percent of Christie’s sales in London, the auction house, like its rival Sotheby’s, said it had no plans to reconfigure its European operations. “We still think London is the best marketplace,” said Dirk Boll, president of Christie’s Europe, Middle East, Russia and India. “And not having any import tax would help.”

For the moment, sellers still express confidence in London as a venue to auction high-value art. Sales of Impressionist and modern art at Christie’s later this month are estimated to raise at least 207 million pounds, or about $270 million, the highest valuation ever placed on an “Imps and mods” season at the company’s London salesroom.