Migrants attempting to enter the UK illegally used to focus their efforts on the French port of Calais.
But following a crackdown there, they have spread their efforts out to ferry ports across Europe, as far afield as northern Spain.
BBC correspondent Colin Campbell spent a week visiting four different European ports to follow their trail.
Many of the people trying to get to the UK are economic migrants.
But an internal Home Office document seen by the BBC shows asylum claims have risen.
That document says the number of migrants waiting for their asylum claims to be processed in the UK is “unacceptably high”.
A note on terminology: The BBC uses the term migrant to refer to all people on the move who have yet to complete the legal process of claiming asylum. This group includes people fleeing war-torn countries, who are likely to be granted refugee status, as well as people who are seeking jobs and better lives, who governments are likely to rule are economic migrants.
Camera: Guy Thompson.
Editors: Samar Salekipour and Greg Brosnan.