Transport for London calculates that 11 million people use the bus or Tube in London every day, of which some 70,000 dodge paying their fares.

That equates to £200,000 a day in lost revenue on the Tube alone and an estimated overall loss to TfL of £116 million a year. Currently about 25,000 fare dodgers are prosecuted, which may sound like a lot of prosecutions but represents a success rate of just over 35 per cent.

In this compelling but depressing behind-the-scenes first episode, we meet Tim, who runs TfL’s control centre and together with his team is responsible for tracking and pursuing any suspicious activity that gets flagged up on the system “as possible criminal behaviour”.

Today he’s looking at the movements of a passenger who has been tapping out at one end of the journey only. “Someone’s attempting to hoodwink the system into believing they’ve made a shorter journey than they’ve actually made,” says Tim, calculating that TfL has lost £1,000 in fares during the year this passenger has been operating.

Using data from the travelcard together with CCTV imagery from the relevant stations, a potential suspect is identified physically. From then on, it’s straightforward detective work for undercover Tube investigator James, who loiters by the exit gates at North Greenwich clutching a mugshot of the suspect on his smartphone, between 6.58am and 7.24am, hoping to nail him if and when he appears.

Meanwhile, on the buses, revenue inspectors Alex and Babs are questioning a woman who has failed to tap in. She is asked to leave the bus and, angrily denying she’s done anything wrong, accuses Babs of making her late. “No I haven’t. You’ve made yourself late,” replies Babs with impeccable good manners. As the woman has no previous record of fare dodging she is issued with a £40 penalty fare but won’t be prosecuted.

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The programme, which tracks several other fare dodgers, shows us not only the shocking scale of the criminality that operates daily on TfL but what these revenue inspectors, revealed here as unsung heroes, have to put up with. They are shouted at, sworn at, abused and even occasionally threatened with violence by these dreadful people yet they remain polite and calm.

Cool heads certainly come in handy when bus revenue inspectors Thomas and Yaw question a passenger who not only refuses to give his name and address but, as he makes a run for it, appears to have a weapon in his backpack.

“Could you do me a Code Red, please?” Thomas asks the driver, urgently. Suddenly, police cars are arriving and a full-scale chase for the man is underway. “Knife crime is an epidemic right now,” Thomas says.

“It takes that one person who’s got that weapon on him and who’s not in that right state of mind, or thinks he’s been disrespected, and we can be … you know, not going home.” He pauses ruefully. “We stay safe every day. Or we try to, anyway.”

Fare Dodgers: At War With The Law is on Channel 5, 9pm tonight