WASHINGTON -- Starting Monday, travelers from 35 countries must request authorization online to enter the United States, the US Department of Homeland Security (DHS) said Friday.

The Electronic System for Travel Authorization (ESTA) has been applicable since August 2008, but will become compulsory from January 12 for applicable European countries, Japan, South Korea, Brunei, Singapore, Australia and New Zealand.

These countries are currently exempt from visa requirements to enter the United States for short visits under the Visa Waiver Program (VWP). The new program will keep travel to the United States "visa free" for travelers from VWP countries, the DHS said.

Instead of travelers filling out paper I-94 visa waiver cards en route to the United States, the new measure requires online registration.

It is a "free service, the approval comes back very quickly ... it is almost immediate," said DHS spokeswoman Kathleen Kraninger.

Once approved, the waiver is valid for years, she added.

Travelers who fail to fill out the online form "might be delayed at the border," Kraninger said.

The new measure is among the recommendations of the 9/11 Commission, which sought to promote laws tightening border security in the wake of the September 11, 2001 attacks.

The DHS recommends that applications be submitted as soon as an applicant begins making travel plans. Upon authorization, travelers can enter the United States for 90 days for business or pleasure.

Instructions on how to obtain travel authorization are available on the website in 22 languages, including English, Danish, Dutch, Finnish, French, German, Icelandic, Italian, Japanese, Norwegian, Portuguese, Slovene, Spanish and Swedish.