Any negative effects for the travel industry will likely be most acute in London, where at least 40 people were killed and more than 700 were wounded by explosions set off in the city's subway system and on a double-decker bus.

Tour operators said that based on recent experiences with terror attacks — most notably the commuter train bombings that killed 191 people in Madrid in March 2004 — trips to the affected region wane somewhat and would-be travelers hold off from making future plans.

Robin Tauck, the president of Tauck World Discovery, an upscale tour operator based in Westport, Conn., said that trips to Spain have experienced "a soft period of sales in 2004 and 2005."

Tauck, which has a group of tourists in London and has other trips planned for later this summer, said no guests have canceled upcoming trips.

"People are watching the news and making their own decisions," Tauck said. That is in stark comparison to the reaction that followed the attacks of Sept. 11, which prompted 11,000 Tauck customers to cancel their trips and 8,000 of them to return home from abroad.

Bob Whitley, president of the United States Tour Operators Association, said members reported yesterday that some travelers scheduled to leave for Europe this week are postponing their trips, though there has been no evidence so far of mass cancellations.

Whitley and other industry officials said the fallout would have been more severe had the attacks affected air travel.

Hospitality and leisure analyst Anthony Rodolakis at PriceWaterhouseCoopers in Tampa, Fla., said the attacks nevertheless will crimp what has been a strong period of growth for London's tourism industry.

Hotel room rates surged by 9.7 percent in April, compared with a year earlier, and the city's hotel occupancy rate climbed 3.6 percent during the same period, Rodolakis said.

With London hotels more than 80 percent full, PriceWaterhouseCoopers had forecast room rates to continue growing at a strong pace in 2005, but now "all bets are off," Rodolakis said.

The stock prices of major airlines fell in response to the blasts, as did oil prices, in part because of the possibility that jet fuel demand would weaken.

Airline analyst Jim Corridore at Standard & Poor's in New York said the skittish reaction from investors was to be expected given terrorism's potential chilling effect on tourism.

However, barring any follow-up attacks, he and other analysts said airline shares would recover quickly.

"We don't expect that there will be a major impact on international travel," Corridore said.

"Bookings today will obviously not be good, but I'm sure they'll pick up again," he added.

Airline analyst Jamie Baker at J.P. Morgan Securities said "given the size of the U.K. tourist market and disruptions to its infrastructure, it is reasonable to expect a slight decline in U.K. booking levels for at least the next six to 10 weeks."