The increasing popularity of a Spanish migrant route has seen three times as many illegal immigrants caught at British port in the last year.
Figures show that officials have already detained 1251 people trying to board ferries heading for Portsmouth, Hampshire, this year up until the start of September compared to 436 in the whole of 2016.
Brittany Ferries, which runs the route from Bilbao to Portsmouth, has has called on port security in Spain to be tightened to stop offenders repeatedly trying to board ships.
A Brittany spokesman said most of those involved are young, male Albanian nationals who are "well-organised" and repeatedly target the port's perimeter in Bilbao.
There are two Brittany sailings each way each week between Portsmouth and Bilbao and the company also runs freight services between Bilbao and Poole.