The European Commission estimates that a permanent reintroduction of border checks in Europe's passport-free travel area could cost up to 18 billion euros ($20.7 billion) a year.

Some EU countries, including Germany, have temporarily tightened border controls as hundreds of thousands of people fleeing conflict and poverty have sought asylum.

The Commission said Tuesday that the direct cost of any permanent move could amount to between 5 billion and 18 billion euros a year.

It would include more expensive road transportation, the impact of delaying and controlling more than 1 billion cross-border trips each year, as well as administrative and customs costs.

The estimate doesn't consider the impact on tourism, lower trade volumes, changes in production chains and the possible undermining of the EU's common visa system.