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Passport checks on travellers leaving Britain are being brought in at the Port of Dover and Eurotunnel in Folkestone from today.
The Government is hoping a phased introduction of the new scheme will cut down on long queues for departing passengers.
But, Eurotunnel has warned of border travel "coming to a standstill'' in future years unless new smarter technology for the checks is introduced.
The Home Office has said the exit checks will improve the Government's understanding of who is leaving the UK and create "a much clearer picture of who is staying in the country when they have no right to be here''.
The checks will eventually see all passengers having their passports scanned and verified, but will be introduced gradually to try to minimise any possible disruption, changes are being introduced gradually.
For the first month, only 25% of the passports will be fully checked, with this percentage moving to 50% after one month and 100% by the middle of June.
As well as at sea ports, the checks are also being introduced at airports - but as airlines will be providing advance passenger information from an individual's travel documents, air passengers will not notice the new system in action.
A P&O Ferries spokesman at Dover said: "They have picked a quiet day for the introduction of the checks. We are hoping that there will be a fairly seamless transition to the new system.''
Eurotunnel said that it was moving to full 100% checks straight away, with the company having spent £2.5 million on new systems, with road markings altered and 50 new staff taken on.
Eurotunnel public affairs director John Keefe went on: "There has been quite a lot of concern about the impact the checks will have, but we are comfortable about going to full operation from today.
"However, over the next five years we are looking at Eurotunnel growth of 20-25% in passengers and 30% in truck traffic. The Government's approach to managing the borders will bring them to a standstill - we need smarter technology.''
Speaking about today's introduction of the checks, a Home Office spokesman said : "Due to the different environments and scale of operations at ports, delivery approaches will vary.
"The Home Office has worked with carriers and ports to allow them to introduce exit checks in a way that aligns with their existing systems, including taking a phased approach at some ports to help minimise the impact on customers and on port operations.''
Speaking last month, Security and Immigration Minister James Brokenshire said: "It is right that we have an immigration system that is fair, that tackles illegal immigration and that clamps down on those who try to cheat the system by staying here when they have no right to do so.
"Exit checks will provide us with vital information that confirms a person's exit from the UK. The coalition Government committed to reintroducing them in 2010 and the Immigration Act 2014 put in place legislation which gave carrier and port staff the powers to carry out these checks.
"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.''