The accession of the two new EU member states - Bulgaria and Romania - has come amid falling enthusiasm in Europe for the bloc's continuing expansion.
A Eurobarometer poll in autumn showed that only 41% of people in the 15 states that were part of the EU before 2004 support further enlargement.
From 1 January 2007 the EU will have 27 members - up from to 15 three years ago - and a population of half a billion.
At a summit in December, European Union leaders said that countries wanting to join will have to meet strict conditions from now on, but that the EU's doors are still open.
The leaders called for reports on the impact of new members on EU policies.
They also said the EU's capacity to absorb new countries would determine the pace of enlargement.