(WARSAW) - Illegal immigration may be the price Europe will pay after Thursday's expansion of the Schengen zone to 24 nations with the midnight entry of nine new members, the continent's border chief said.

"What is our concern in terms of Schengen is that we are going to lose a very effective instrument to fight illegal immigration," Ilkka Laitinen, executive director of the Warsaw-based Frontex EU border watchdog told AFP.

Eight former Communist states and Malta -- all fledgling EU members -- enter the Schengen zone at midnight, creating a vast visa-free area for 400 million Europeans in 24 countries stretching from Spain to Estonia.

Once people enter the zone, whether legally or otherwise, they would be free to move across all member states.

Laitinen said European nations were aware of this but "it is a deliberate choice of the European Union to focus more on the free movement of persons than on security aspects."

However, corruption along the zone's 5,000-kilometre-plus (3,125-mile-) eastern frontier with Russia, Belarus, Ukraine and northern Balkan states is "a persisting and permanent risk" for Schengen border security, Laitinen said.

He said the widespread use of highly reliable biometric identity instruments could spark "stronger attempts to blackmail or corrupt law enforcement" officials by criminal organisations smuggling people and goods into the zone.

"When there are less possibilities to cross the border illegally, you see blackmail or corruption. The situation will change in this way," says Laitinen.

"Corruption is not only taking place at the border. In many cases, it is a systematic phenomenon which goes from top to bottom," the Frontex head said.

"The criminal organisations will try to find other ways to pass their things through. Corruption is one of the most probable modus operandi in this regard."

The EU border chief however stressed that the expanded zone would not result in a new wall dividing Europe.

"I've heard that the Schengen acquis would be a key problem to promote bona fide travel. I do not subscribe to this kind of declaration. We have a lot of examples where the Schengen visa regime has increased the cross-border volume, for instance between Finland and Russia," Laitinen said.

A former Finnish border guard, Laitinen said mobility will depend on how well prepared new Schengen zone entrants are to administer and implement the visa regime.

"If they do not invest enough in human resources or financial resources for offering visas in due course for those who are applying, if they don't have enough staff to carry on the border checks, there will queues and the border will be less simple to cross," Laitinen said.

"But if they really take this as a challenge and work accordingly, there are all possibilities to succeed."

After two years of preparation, 2004 EU newcomers the Czech Republic, Estonia, Hungary, Latvia, Lithuania, Malta, Poland, Slovakia, and Slovenia will join the 15 oldest EU states -- bar Britain and Ireland -- plus Iceland and Norway in the zone at the stroke of mid-night December 20.

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