Travelers from the 35 countries are asked to obtain an authorization from the U.S. Department of Homeland Security at least 72 hours before their flight departs to the U.S.

The program has been voluntary since August and becomes mandatory Monday.

U.S. Embassy spokesman Dick Custin in Britain said Friday that approval has been granted in nearly all of the cases involving voluntary participants.

The online application form asks tourists to say if they are drug users and divulge certain criminals convictions or past terror activities.