Britain's European neighbors decided today to tolerate that country's softened positions they have pledged to impose this week.

West Germany and France formally announced that orders to cut off trade contracts with Iran dating back to last November's seizure of hostages at the U.S. Embassy in Tehran would go into effect after tomorrow. The remaining six European Community members are expected to take similar action tomorrow.

Representatives of the Common Market countries met in Brussels today to discuss the impact of Britain's sudden decision to limit its sanctions to a ban on future contracts for trade with Iran.

No official statement was released after the meeting. But knowledgeable sources said the European allies, while annoyed with Britain's breach, pledged to honor their previous commitment on sanctions in the interest of keeping their word to the United States and preserving European solidarity.

What reportedly worried the European officials most was the possibility of British firms undercutting other European companies by snapping up abrogated Iranian contracts. Concern was also expressed about the avenues for easy evasion opened by the British withdrawal.

Britain's own order is not expected to take effect until next week, allowing time for West European companies to evade their own government restrictions by using British connections to close additional deals with the Iranians.

But sources said these worries were eventually dismissed as not compelling enough to force an unraveling of Western Europe's previously unanimous decision to impose sanctions.

U.S. officials in Brussels greeted this news with relief. They had feared that Britain's action would give other Western European nations a pretext to drop their own Iranian embargo plans -- which were accepted in Naples Sunday only with great reluctance.

The West European states have been anxious to coordinate their approach on Iran so that no single country goes further or makes more of a sacrifice than any other.

Britain's own trade with Iran has lately been on the rebound, jumping back to more than $100 million a month, nearly as high as before the Islamic revolution there.

Much of the rest of Western Europe's trade with Iran also has been recovering. At the start of the year, Common Market trade with Iran was on a rising curve at around $500 million a month.

One reason the West Europeans may be willing to hold to their embargo pledge is that it will affect less than 10 percent of their Iranian business, according to a Dutch estimate that is the only official figure offered so far.

The great bulk of West European trade with Iran originated with contracts signed before the taking of the hostages and thus is unaffected by the ambargo.

Further, the retroactive ban applies simply to trade contracts. It does not apply to service contracts, including lucrative consulting arrangements, engineering deals and the maintenance of industrial projects.Only new service contracts will be prohibited.

The Europeans, like the United States, still will be able to trade freely with Iran in food and medicine. This is especially significant for France, which does much of its business with Iran in food, and for Denmark, whose largest single export item to Iran is feta cheese.

Given all the economic room remaining for Europe to maneuver with Iran, the ultimate effect of Britain 's reversal may be to slacken the enforcement effort of other European governments once their bans are put into effect.

But the British move struck its blow to Europe's developing sense of cohesion. In heavy understatement, the West German spokesman, Klaus Boelling, said today, "The federal government regrets that as a result [of the British action] the unity of the Naples decision has been impaired."