WASHINGTON - The United States said yesterday that visitors from closely allied countries such as Britain and Japan soon would be required to register personal details online at least three days before arrival.

Homeland Security Secretary Michael Chertoff, who announced the changes yesterday, said they would help the United States boost the security of its visa-free travel program by allowing the government to screen visitors before they arrived.

Currently, visitors fill out paper forms en route and are screened by U.S. customs agents upon entry.

The government will begin implementing the changes in August, Chertoff said. Online registration will be mandatory for all visa-free travel by Jan. 12.

Currently, citizens of 27 countries are not required to obtain visas for U.S. entry, including most of Western Europe, Andorra, Australia, Brunei, Japan, New Zealand and Singapore. Eight other countries - Czech Republic, Hungary and South Korea among them - are expected to be admitted to the visa-waiver program.

When the Homeland Security Department began discussing the online-registration rule last year, European businesses expressed worry that business travel could be impeded.

But Chertoff maintained that the system would simplify visa-free travel, because visitors would be required to register online only once every two years rather than to fill out forms each time they traveled.

Britain, whose citizens have long enjoyed visa-free travel to the United States, said it would accept the changes.

"Obviously we understand the need to improve travel security, and we welcome this additional information from the U.S. on when and where they are going to implement [the new measures]," a British Home Office spokeswoman said on the government's customary condition of anonymity.

The Confederation of British Industry, a business coalition, said it was reassured that the measure contained provisions for last-minute business travel.

Ahead of the announcement, European Union officials said they had been discussing the changes with Chertoff. Earlier this year, the EU unveiled plans for a similar travel-authorization system for citizens from countries that had a visa-free regime with the EU, but details have yet to be worked out.