Naturally there are no reliable figures on this phenomenon. Estimates of the number of migrants who are not in possession of regulated residence and/or work permits are mostly based on the number of persons seized. According to one current estimate, there are between 2.8 and 6 million persons in the EU who do not have regular residence status.
If we consider the large number of regulations, measures and high level of cooperation in this area, then, without a doubt, the fight against irregular immigration can be regarded as the focal point of European cooperation in matters of migration. Even the 1990 Convention implementing the Schengen Agreement contained strict rules on checks at external borders, provisions concerning penalties for haulage companies that transport people with no entry documents, as well as restrictive visa regulations. In addition, cooperation in matters of repatriation and migration control with countries of origin and transit forms a focal point of measures against irregular migration. The Council of Ministers has yet to agree on a 2007 European Commission proposal for a Europe-wide directive intended to harmonise the prosecution and penalising of employers who employ irregular migrants from third countries.
To a certain extent, as compensation for the symbolic loss of controls on internal borders, provisions concerning checks at external borders have been particularly far-reaching, crossing over, among other things, into the foundation of the European border security agency, Frontex. Frontex, brought into being in 2005 as an agency to coordinate national border patrols, has seen its scope of activity constantly expanded. The creation of rapid border intervention teams (so-called "Rabits") merits special mention; these may be regarded as the forerunners of a possible European border police. The expansion of the border security agency's operative capacities and of its budget from EUR 19 million in 2006 to EUR 70 million in 2008 points to the political priority member states give to securing the external border. This priority setting adds fuel to complaints that the EU is building a "fortress Europe", an accusation made in scientific literature as well as by numerous non-governmental organisations.
A further dynamic focal point of measures to combat irregular immigration concerns cooperation with countries of origin and transit as well as repatriation policy. The Schengen states recognised early on the advantages of a coordinated procedure for matters of repatriation by concluding a multilateral readmission agreement with Poland in 1991. The Treaty of Amsterdam transferred the power and responsibility for concluding readmission agreements with countries of origin or transit to the European Commission. The insistence on establishing an obligation in the agreement to readmit even persons who are neither citizens of an EU member state nor of the third country concerned, however, has considerably limited the success of negotiations for such agreements. Since under international law a country is only obliged to readmit those of its own citizens who have been staying illegally in another country, to date important transit states such as Morocco have resisted concluding such a comprehensive readmission agreement. By contrast, it has been possible to secure the agreement of eastern European neighbours such as Ukraine and Russia in exchange for eased visa issuance procedures.
Repatriation policy is embedded in a more comprehensive cooperation with countries of origin and transit, which has become the true focal point of European cooperation in matters of asylum and immigration policy, especially since the 1999 European Council meeting in Tampere.