Organizers insist fans will be allowed into games only with a ticket that bears their name and ID card or passport number. The measure, recommended by the European Union, is designed to exclude hooligans and curb scalping.

But an official said Monday that the holders of some tickets sold to companies or given to sponsors for VIP guests had yet to supply the information ¡ª and it might be too late to fix the problem.

"We are in talks with the other responsible parties in search of possibilities to bring down as far as possible the proportion of tickets that can't be personalized before the various games," Interior Ministry spokesman Stefan Kaller said.

Kaller said less than 1 million of the 3 million tickets for the June 9-July 9 tournament were potentially affected.

Organizers are allowing fans to sell back tickets or change the name printed on them for a fee. They plan spot checks at stadiums to deter anyone from buying tickets from scalpers.

The World Cup Organizing Committee said personalizing all tickets remained "fundamental" to its security concept.