Train operator Thameslink is recruiting 60 plain-clothes ticket inspectors to crack down on fare dodgers.

The undercover inspectors will target cheats who hide in toilets or get off when they see uniformed inspectors.

The company, which operates services between Brighton and Bedford, estimates fare dodgers cost it £4 million a year.

It believes they include regular commuters with jobs in London who persistently evade paying.

The team will dress in office-wear in the morning and casual clothes later in the day.

One of the fraudsters' tricks is to buy a cheap ticket to a station just up the line in order to get through automatic barriers.

They then travel to a station without automatic barriers or regular ticket inspectors.

The team will work closely with British Transport Police and follow targeted fraudsters for entire journeys, changing trains if necessary.

Thameslink director and general manager Mark Causebrook said: "There is a hard core of persistent fare dodgers that cost vast sums of money and are difficult to detect by conventional means."

The undercover inspectors will also be available to help control trouble on trains.

Mr Causebrook said: "Unruly behaviour on the trains occurs from time to time and ranges from schoolchildren interfering with the workings of the trains through to late night revellers who have enjoyed themselves too much.

"We feel it is important to control these situations for the benefit of all customers."