Europe's challenge to the American ban on shipping equipment for the Trans-Siberian gas pipeline was heightened today with the announcement that a Soviet freighter will arrive in Britain this weekend to pick up turbines manufactured with U.S. parts.

In Bonn, a spokesman at the Economics Ministry confirmed today that the government has sent letters to West German firms involved in the pipeline project urging them to honor their supply contracts despite the U.S. ban.

A Soviet ship is already in France to be loaded with three compressors made by Dresser France, a subsidiary of an American firm. Loading was postponed today but was expected to take place Thursday, marking the first direct violation of President Reagan's restrictions on supply of the pipeline equipment.

This evening, a spokesman for John Brown Engineering Ltd. said that a Soviet freighter would arrive in Glasgow, Scotland, Sunday or Monday to receive six turbines using rotors manufactured by General Electric. The West German government, which consistently has supported the pipeline project, lacks the legal power to force defiance of the U.S. embargo, but the letters, sent earlier this week, encouraged the firms to press ahead with their participation.

In Brussels, Gaston Thorn, president of the Commission of the European Community, called for talks to avert a crisis. "We must at all costs avoid an escalation and avert a trade war, which would be damaging to both sides," he said. But informed diplomatic sources in the European capitals involved said there appeared to be no moves left to keep the confrontation from going forward.

While the Europeans are anxious to avoid excessive recriminations, one senior American said, "They have obviously decided that the equipment will be sent." Thorn's appeal for further talks was personal and did not reflect a new European initiative to resolve the impasse, officials in Brussels said.

With the loading of ships in France and Britain now in prospect, the main question in Europe is what sanctions the United States will invoke against the companies defying the ban. On Monday, a federal judge in Washington refused to block the administration from imposing penalties on companies that defy Reagan's order and Secretary of State George P. Shultz met with other top officials to plan such measures.

While the European countries involved -- Italy as well as France, Britain and West Germany -- are agreed that the ban is unjustified and should be defied, there are differences in the nature of the orders to the companies. This ensures, sources said, that the issues will become enmeshed in legal and technical problems that could last for years because not all the companies can be treated the same way.

In the meantime, they said, the bulk of equipment will be shipped, the pipeline constructed and political damage to the Western alliance from the controversy is likely to prove the most serious lasting consequence. "With things having gone this far," one senior diplomat said, "it's hard to see how the U.S. can avoid shooting itself in the foot."

Of the options open to the United States, the most attractive in terms of minimizing the long-term effects on U.S.-European relations would be, as one source put it, to relegate the subject to "the indefinite sands of legal wrangling." But diplomats in a number of capitals, Europeans and Americans alike, said they had no idea what the American response would be.

There was no explanation this evening for the delay in the loading of the three Dresser compressors, part of an order of 21, that are already at the port of Le Havre. Reuter quoted French Industry Minister Jean-Pierre Chevenement as saying, "a compromise is always possible, but I am not aware of any compromise at this stage."

Some officials suggested that the French still may be hoping that the United States will back down at the last minute, but there was no evidence today that is at all likely.

Officials at John Brown refused to discuss details of the Soviet ship's expected arrival. After the British government earlier this month ordered the firm and three others to ignore the U.S. ban, John Brown spokesmen said delivery of the six turbines would be completed by the end of August.

It is not clear, however, whether the company will be able to fill the balance of its almost $200 million order, the largest British order for the pipeline. The company had GE rotors for the six turbines in stock last December when Reagan imposed the first phase of his ban after the martial-law crackdown in Poland. But rotors for the remaining 15 turbines will have to be obtained elsewhere.

The dilemma for the French and British firms is that they face possible punishment from their governments if they accept the American restraints. Dresser officials said in Washington Monday that the French penalties could be as much as $60,000 and 12 months in jail.

The British companies are acting in response to a new law, the Protection of Trading Interests Act, which has never been used before. British officials concede they are not certain exactly how the law would be enforced should a company at any stage accept the terms of the American ban to avoid, for instance, blacklisting in the United States.