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.
“Asylum intake for October, November and December 2018 hit the highest levels since October 2015… this is unacceptably high,” said the leaked document. ”It risks reputational damage to the Home Office and Asylum process.”
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.