Dozens of migrants were detained on Sunday in the French port of Calais, trying to board a cross-Channel ferry bound for Dover.
French police arrested more than 40 people after they scrambled aboard a ferry docked at the port overnight. Twelve people are due to appear in court; the others were released with a warning.
The migrants broke into a highly protected area and used ladders to climb onto the ship. Cross-Channel traffic was delayed as police launched a 12-hour search for migrants hiding aboard.
Euronews correspondent Anelise Borges says migrants are trying to get to the UK before March 29, when Britain is set to leave the European Union.
"Reports of what a no-deal Brexit could mean for security checks in the port area has had an impact on the plans these people make," Borges told Belle Donati on Good Morning Europe. "They say they are very worried about not being able to reach the UK so several of them are trying to hasten that plan, trying to get to the UK as soon as possible."
Authorities say they don't know if the incident was organised or spontaneous but it has raised concerns about port security in the run-up to Britain's exit from the EU.
"We must reinforce our police forces on the eve of Brexit which people smugglers exploit in a bid to promote their trafficking," Xavier Bertrand, president of the Hauts-de-France region that encompasses Calais, said on Twitter.
French and British authorities have already boosted coastal security, but migrants and smugglers have found new ways of trying to cross the Channel.
Many hide in trucks headed to French ports, but recently others have attempted to cross the Channel in dinghies and lifeboats stolen from local harbours or acquired by traffickers.
Last week, a French court jailed two Iraqis and an Iranian for organising illegal migrant boat trips to Britain.
A 30-year-old Iranian, considered the group leader, received an 18-month sentence, while his two accomplices, aged 39 and 32, were each jailed for a year.
Last year saw a huge spike in migrants attempting the often dangerous sea-crossing. Around 500 people tried to make the journey, compared to just 13 known attempts in 2017.