The head of the European Union's border watchdog has warned of a possible rise in illegal immigration because of the enlargement of the Schengen area.
Ilkka Laitinen says free movement has been put above security
Executive director of Frontex, Ilkka Laitinen, was speaking hours before nine EU countries were due to join the passport-free zone.
The scrapping of border controls will affect an estimated 400 million people.
"We are going to lose a very effective instrument to fight illegal immigration," he said.
As soon as people had entered the Schengen zone legally or illegally, he said, they would be free to move across the entire area.
Mr Laitinen said European countries were well aware of the potential problem but it had been "a deliberate choice of the European Union to focus more on the free movement of persons than on security aspects".
Similar concerns have been expressed elsewhere.
The head of the German police union, Konrad Freiberg, has said the problem of people trafficking will become acute.
And 75% of Austrians questioned for a television poll said they opposed the lifting of barriers.
The UK, which is not part of the Schengen area, has revealed it has stepped up security and intelligence-led operations.
The immigration minister, Liam Byrne, said the measures had been co-ordinated with France.
"We are ordering extra searches, and extra security, alongside our attack on organised crime," he said.
"I apologise if queues get a little longer but tougher checks take time."
The Austrian Chancellor, Alfred Gusenbauer, rejected fears of increased crime as a result of the scrapping of controls.
He said the accord was not about criminality, insecurity or fear but it was instead "a bigger zone of peace, security and stability".
The European Commission President, Jose Manuel Barroso, was also upbeat.
"Together we have overcome border controls as man-made obstacles to peace, freedom and unity in Europe," he said.