WASHINGTON — American officials, hoping for a tougher line from Europe and Britain, fear the Iranian government will hold hostage the 15 royal marines seized on March 23 until Prime Minister Blair is out of office this summer.

That was a concern this week as Iran hardened its line in negotiations, demanding an official apology from London and airing an alleged apology from one of the seamen captured last month, the second such televised confession of a British captive.

Yesterday, an American intelligence official said the combination of the provocative aired confessions and changing demands made in diplomatic channels has led the Pentagon and military to conclude that the Iranians intend to drag out any negotiations for the release of the hostages until Mr. Blair is out of office, a move in line with the negotiation tack favored by the mullahs in the hostage crisis with the Carter administration. The Iranians waited until after the 1980 election to release the diplomats they had held for 444 days.

While Mr. Blair has not yet announced when he will step down as prime minister, the British press wrote in September that he is preparing to step down as the head of the Labor Party on May 31 and will leave 10 Downing Street on July 26. If the Iranians hold onto the hostages until Mr. Blair is out of office, it would also be a blow to American prestige, as Mr. Blair was the most vocal ally of America’s war to topple Saddam Hussein.

The concern that the mullahs are playing for time was influenced in part by promises by the Iranian foreign ministry to begin a show trial of the 15 sailors in the coming weeks. Through diplomatic channels, the Iranians have demanded the release of five members of their Quds Force taken hostage. That could set the stage for a swap. But the demand for a hostage exchange has also varied widely depending on the diplomatic channel.

“There is no hard intelligence right now on the motivations behind this,” the intelligence official told the New York Sun yesterday. “It is an informed analysis that circulated in the intelligence community. It is one of the explanations thrown out to explain what they are doing right now.”

Other possibilities discussed in the analysis include a response to the vote in Congress to set a date for withdrawal from Iraq and a warning to other great powers that have recently cooperated with the United Kingdom and America diplomatically and financially on sanctioning the Islamic Republic for its defiance of the International Atomic Energy Agency. This source yesterday said, “No conclusions have been made on this.”

The British defense minister, on a trip in Afghanistan, which borders Iran, said that his country was taking the diplomatic route for now. In an interview yesterday with the BBC, Des Browne said, “There is no reason to continue to keep them there. We are anxious that this matter be resolved as quickly as possible and that it be resolved by diplomatic means and we are bending every single effort to that.”

One such effort was scuttled on Friday when Germany, the European country that does the most trade with Iran, refused a British request to suspend trade with the Islamic Republic. Mr. Blair over the weekend condemned the airing of the public confessions of the sailors. President Bush demanded Saturday from Camp David that the Iranians release the hostages, calling the seizure “inexcusable.”

The British people appear to favor a diplomatic route for winning the release of the hostages. A poll published Sunday by the Sunday Telegraph found only 7% of Britons favored military action today. Of those polled, 44% favored military action as a last resort, while 48% opposed military action as a last resort.

In Tehran yesterday, local press reports said 200 people protested outside the British embassy chanting “death to Britain,” and “death to America.” The regime has cracked down on almost all protests against it, and the protests yesterday are likely to have been coordinated in part with the state and its allied militia known as the Basij.