New rules to check the details of all travellers leaving the UK were brought in under the Immigration Act 2014
Passport checks on all travellers leaving the UK are being phased in at ports and border crossings.
Immigration officials will record details of all passengers leaving by commercial air, sea and rail transport.
The exit checks will particularly affect travellers on cross-channel ferries and the Channel Tunnel.
The government said the exit checks were needed to combat illegal immigration and would be introduced with the "least possible disruption".
In an attempt to minimise any possible disruption the exit checks are being introduced gradually.
For the first month all passports will be scanned but only 25% of the passport holders will have their details verified to ensure they are genuine.
After a month, there will be a move to 50% verification and by the middle of June 100% verification will be introduced.
What are exit checks?
Data will be collected on all passengers as they leave the UK on scheduled commercial international air, sea and rail routes.
The government says exit checks are "predominately an immigration and data tool", giving a "comprehensive picture" of whether people leave the UK when they are supposed to.
It says the data - gathered by airline, rail or ferry operator staff - will "improve our ability to identify and further tighten the immigration routes and visas that are most vulnerable to abuse".
It will also help security services "track the movements of known or suspected criminals and terrorists".
A spokesman for P&O Ferries at Dover said the company was "hoping for a fairly seamless transition to the new system".
"They have picked a quiet day for the introduction of the checks. The actual scanning of passports is quick, it's the verification that takes longer," he said.
Security and immigration minister James Brokenshire said: "Exit checks will provide us with vital information that confirms a person's exit from the UK.
"Port and travel operators are experts in their business and know their customers best, which is why we've supported them to design and trial the systems for collecting data in a way that will minimise the impact on customers."
The new rules were brought in under the Immigration Act 2014, which gave carrier and port staff the powers to carry out checks.
The government says a "piecemeal" paper-based system of embarkation checks used to operate, but was scrapped entirely in 1998.
The Advance Passenger Information system, introduced in 2004, gives the government information on passengers flying in and out of the UK - and exit checks mean information will now be gathered on people leaving by any commercial transport.
People arriving in the UK remain subject to passport checks.
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