The Port of Dover is asking ferry passengers to come prepared for delays, as the half term getaway begins.
It is the first holiday period since the EU's new border system was fully implemented.
Since 10 April, the Entry Exit System (EES) was meant to be in operation across all external borders of the Schengen free movement area, which includes many popular holiday destinations like France and Spain.
However some countries aren't yet taking fingerprints and photos.
At Dover, where people go through the French border before they board a cross-Channel ferry, French authorities have not yet switched on the machines which will take fingerprints and photos under EES.
However, border officials still have to do part of the process for each tourist. Setting up a profile linked to the new system means it will take a little longer to get through.
The port says if passengers miss their intended sailing because of delays, they can get the next available departure.
Some 18,000 cars are expected at Dover between Friday and Sunday, with Saturday the busiest day. Four hundred coaches are expected on Friday alone.
The Lydden hill car racing track will be available as a contingency measure to hold cars if queues get really bad, to avoid local roads getting clogged up.
Border authorities are allowed to suspend EES altogether if severe delays build up.
The Port of Dover is asking people to:
At the Channel Tunnel operator Eurotunnel's Folkestone terminal, French authorities also haven't yet turned on the new machines to take car passengers' biometric details.
The situation is similar to Dover, in that border officials in booths are creating "profiles" for tourists, but fingerprints and photos are not yet being taken. It's not warning passengers of delays.
Lorry drivers have been providing biometric information for a while now.
Eurostar's hub at London St Pancras station is the other UK location where French border checks are done as people leave the country. Again, border police have been creating files, and in some cases taking biometric information. But most passengers haven't yet used the automated machines.
Greece has said British passengers won't face biometric checks this summer.
At some airports, passengers have experienced long queues at the border upon arrival in recent months.
In April, about 100 people were left stranded in Milan after a flight to Manchester left without them.
On Thursday the boss of Easyjet, Kenton Jarvis, told the BBC problems caused by the EES were "unacceptable".
He said "we have seen some reduction in some of the queues".
However, he encouraged European countries to use the flexibility they had to go back to manual passport stamping if necessary.
This week, Easyjet and Jet2 have tried to reassure customers they can book with confidence, and they have no fuel supply issues.
A trend has emerged of people booking their holidays late, because they're nervous about the impact the Middle East conflict could have on the future availability of jet fuel.
Travel association ABTA said demand was particularly strong for trips to Spain, Italy, the Balearic and Canary Islands, Portugal, Croatia and Greece, with Majorca, Alicante and Tenerife proving popular for beach holidays.
It said people were prioritising value and peace of mind.
When it comes to traffic on the UK's roads, the RAC thinks this late May bank holiday weekend will be the busiest in two years, with almost 19 million getaway trips being made.
