Here are some key questions about the process.
Which vehicles are checked?
Lorries, containers, other freight vehicles and cars can be subject to checks as they travel to the UK.
Not all vehicles are checked when they board and decisions by Border Force officers on which ones to search are “targeted” and based on intelligence, the Home Office said.
Border Force works with the National Crime Agency and European authorities in France, Belgium and the Netherlands to share intelligence to target organised crime gangs behind people trafficking and smuggling, the Home Office said.
Where are checks made?
The UK has border controls in France and Belgium, where it carries out passport checks of travellers as well as searching for smuggled goods.
The checkpoints are in ports at Dunkirk and Calais, the Eurotunnel terminal in Coquelles, and at Calais-Frethun and Brussels Midi stations for Eurostar passengers.
Are there sufficient resources?
The Home Office has insisted there are enough resources made available to police ports and other borders.
It said it is now in the process of hiring 1,000 more staff to “help maintain security” at borders.
In the last year, 900 officers were hired.
The Home Office said it is “continually looking to improve our capability to respond to changing threats, to detect, deter and disrupt harm”.
But Mark Serwotka, general secretary of the Public and Commercial Services union, said: “The deaths of 39 people trapped in a lorry trying to enter Britain for a better life are an absolute tragedy.
“Our members working at the borders have suffered staffing cuts and are strapped of resources to be able to do their job in an effective and humane way.”
What kinds of checks are carried out?
Officers can use carbon dioxide detectors, motion sensors and sniffer dogs to detect people hiding in lorries.
Containers are identified at ports to be searched by officials.
When searching cars, officers may test surfaces for drug residue, check underneath the vehicle and manually search among luggage in the boot.
What responsibility do drivers and haulage companies have?
Drivers and hauliers are told to “operate effective security measures” to stop migrants hiding in their vehicles to enter the UK illegally.
If any are found, the drivers and companies can be fined.
But some lorry drivers have told of daily difficulties in trying to prevent their cargo being targeted, even when they have taken steps to secure their vehicles.