By Daniel Sandford
Home affairs correspondent
A suspect is discreetly arrested at Heathrow's passport control
The e-borders system that counts people in and out of Britain has now processed 100 million journeys, with 3,700 arrests. It aims to cover 95% of all journeys by the end of 2010.
The BBC has been given exclusive behind-the-scenes access at Heathrow airport to witness e-borders in action.
Our day started early in the control room at Heathrow police station. Many flights which generate alerts from the new e-borders system land in the morning.
In the corner of the control room there is a fax machine linked to the Joint Border Operations Centre (JBOC) down the road.
UK E-BORDER SYSTEM
Scheme first piloted in 2004 but ramped up this spring
100 million journey processed
3,700 arrests made
Aims to cover 65% of all journeys by end of 2009
Aims to cover 95% of all journeys by end of 2010
Hopes to be fully operational by March 2014
When we arrived just after 0700 (BST) there was only one alert on the system. It was a man who had failed to pay some court fines, and it was decided to let him through and tell his local police force that he was back in the country.
The police grade the alerts they get with a simple colour-coding system.
Red means the passenger will be arrested on the plane or at the door of the plane. This is clearly disruptive and uses a lot of officers, so it is saved for cases where there is a risk of violence, or where the alleged offence is so serious police are determined to prevent an escape.
An amber alert leads to the passenger being arrested at passport control in full view of the queue.
Green is saved for alerts where police at Heathrow will not make an arrest, but will pass the information on to the local force.
It looked like it was going to be a quiet day, so we went to see the JBOC. Temporarily based at Heathrow, this is where the lists of passengers arriving in Britain are processed.
The BBC's Daniel Sandford witnessed two intelligence-based arrests
This is the most controversial part of the e-borders project. Many airlines are now being asked to send passenger lists, along with all the passport details, to the UK Border Agency at least half an hour before the flight is due to depart.
By the end of 2010 all commercial flights, and all rail and ferry services, will be asked to provide passenger lists. The minimum requirement is each passenger's name and all the details in their passport.
But the UK Border Agency has the right to ask the carrier for much more information, including their email address, credit card number, car registration number and the name of the person who made the reservation.
Officials say they will only ask for more detailed data on high-risk routes. The passenger lists will all be kept online for five years and in the archive for 10.
Privacy and civil liberty campaigners say this is wildly over the top, but have lost the argument, at least for now.
The JBOC is based in an office on an industrial park. UK Border Agency officers and police pore over the lists, checking them against their databases, including the Police National Computer (PNC).
It is these checks, and the gleaning of other intelligence, that generate the alerts at Heathrow police station and other airports running the e-borders system.
Its beauty, from the police point of view, is that they often have plenty of time to make those checks as long-haul flights are in the air for several hours.
While we were there, the JBOC generated its first serious alert of the day for Heathrow's police.
We accompanied officers as they went to arrest the suspect at the door of the plane. He was a man whom the PNC suggested had failed to appear in court for an alleged assault.
Usually he would have been graded amber, but there was intelligence he may have used violence against the police in the past, so he was graded red.
Four officers were dispatched to meet his flight from northern Cyprus. With pistols on their hips, they waited on the automatic pier as it went out to meet the aircraft and prepared to board the plane.
But the cabin staff urged them not to and delivered the wanted man into the officers' hands while they waited at the door.
He was swiftly led up the ramp, taken aside, and arrested. He could not quite believe they had taken such action just because he missed a court appearance.
He was on his way back from his wedding, accompanied by his wife and some of the guests, and he was due to fly out that night for his honeymoon.
Instead he was going to spend the night in the cells and appear before magistrates the next morning.
The arresting officer was only mildly sympathetic. He said: "It's a rocky start, but not the end of the world is it?"
The next arrest was at passport control. While we were watching the first operation, a new alert had come in. It was another man accused of failing to attend court, this time for threatening behaviour in London's Leicester Square.
The PNC also had him down for breaching a court order. He had been graded amber because there was no suggestion he would be violent towards the police.
We watched as he stepped up to the passport desk, and was then asked by the immigration officer to take a seat.
Two police officers approached and beckoned him forward, took him aside, handcuffed him discreetly and led him away. Two wanted men were behind bars.
Before e-borders began, the technology simply was not there for police to use entry points into the country as reliable pinch points for arresting wanted people.
Murderers on the run probably had something to fear, but if you had failed to appear in court for a relatively minor offence you could still go on holiday with impunity. No longer.
The UK Border Agency and the police now have unprecedented advance knowledge of who is entering and leaving Britain, and the system is only just gearing up.