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 1,251 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.
In Bilbao, a port infraction is treated as a civil misdemeanour and offenders are simply released to try again.
Brittany Ferries spokesman
Brittany Ferries revealed 99 detentions have also been made by staff in the port of Bilbao, Spain, or in Portsmouth or Poole, in the first two weeks of this month.
The Brittany spokesman added: "Bilbao remains an attractive target for criminal gangs.
"In neighbouring Santander, those who repeatedly target port facilities are sent to a detention centre near Madrid before being deported.
"In Bilbao, a port infraction is treated as a civil misdemeanour and offenders are simply released to try again."
Last month Spanish newspaper El Pais reported that a major factor in the increase in detainments was the dismantling of the so-called 'Calais Jungle', where many foreign nationals had set up camp.
A Home Office spokesman said: "Working with our law enforcement partners at home and abroad we use intelligence to keep our borders secure.
"We have also invested in new detection technology and are working with port operators to improve the controlling, screening and processing of unauthorised arrivals."
Portsmouth City Council leader Councillor Donna Jones said: "We are very grateful for the efforts made by Brittany Ferries and Border Force to tighten security around ports to ensure people are not entering the UK illegally.
"The council has the responsibility to look after unaccompanied asylum-seeking children so any entering the country illegally, without a genuine need for asylum, place an additional burden on our already limited financial resource."