WASHINGTON — Even as it bowed to complaints about vacation-ruining passport delays, the Bush administration insisted yesterday it is pressing ahead with restrictions next year that could mean even bigger travel headaches.

Responding to protests, the State Department and Homeland Security Department temporarily relaxed a rule requiring passports for air travel returns from Canada, Mexico, Bermuda and the Caribbean.

From now until the end of September, they said, travelers will be allowed to fly back from those destinations if they present government-issued identification, such as a driver's license, and a receipt from a State Department Web site showing they had applied for a passport.

The reprieve applies only to those with applications pending, not those who apply in coming days for travel later this summer. Travelers with receipts but not passports should expect extra scrutiny.

The goal is to allow more time to process a flood of passport applications that have come in since the rule, one of many enacted after the Sept. 11, 2001 terrorist attacks, took effect this year.

Despite that, security officials said they will proceed, as of January, with a requirement that passports be presented at all U.S. sea and land border crossings. Homeland Security plans to offer a draft in two weeks that spells out how the new rule will be implemented, said spokesman Russ Knocke.

Several members of Congress were incredulous, and pledged to force a change in the January deadline.

"They have got an awful mess that they can't handle and they're going to have to put this whole notion off for another year," said Rep. Louise Slaughter, D-N.Y., whose district lies along the U.S.-Canadian border. "If there is a mistake to be made, I'm sorry to say DHS will make it."

Sen. George Voinovich, R-Ohio, said Congress had given the State Department the flexibility to wait until June 1, 2009, for the land and sea passport requirements. He strongly urged officials to wait "if it looks like what happened this summer becomes a possibility this winter."

The application surge is the result of the Western Hemisphere Travel Initiative, in effect since January, which requires U.S. citizens to use passports when entering the United States from Canada, Mexico and the Caribbean by air.

Maura Harty, the assistant secretary for consular affairs, acknowledged that the department did not adequately anticipate "the American citizens' willingness and desire to comply" with the rule "in the timeframe that they did."

Last year, the agency processed 12.1 million passports. This year, officials expect to process about 18 million, she said. The department received 1 million applications in December, 1.8 million January and 1.7 million in February.

Harty said the department hired 145 people last month to work on the backlog and will hire 400 more this quarter. Target turnaround times for passports were bumped up from six to 12 weeks after the surge, but 500,000 applications have already taken longer, she said.

Those numbers pale in comparison to what lies ahead.

According to government estimates, about 6 million Americans will need formal documents to travel to the Caribbean, Canada or Mexico by air or sea. The estimated need for land crossings is more than four times that: 27 million Americans over the next five years. Those numbers do not include the regular year-to-year demand for passports. The State Department is still working on creating a cheaper passcard alternative for land crossings.

Lawmakers, who had been pushing for a change in the passport rule for weeks, say they are exasperated that it took so long.

Sen. Charles Schumer, D-N.Y., called on the State Department to return the $60 fee it collects for expedited passport applications, since many of those are also trapped in the backlog.

"It's very simple — if you don't get the service you paid for, then the State Department should give you a refund," Schumer said.