Train companies have admitted overcharging thousands of passengers caught fare dodging.

Industry body the Rail Delivery Group (RDG) says eight train companies, including Govia Thameslink Railway and London Northwestern, punished ticketless travellers too harshly by not implementing a rule change made in April last year.

The new legislation reduced the correct punishment for some fare dodgers depending on the time of their journey.

Up to 10,000 passengers could have been affected by the mistake, which was only discovered last month and led to tens of thousands of pounds being unfairly demanded by ticket inspectors.

London Northwestern Railway estimates it overcharged 2,700 people by a total of £12,000.

Sean McBroom, head of on board for London Northwestern Railway, said: "Following incorrect guidance issued in April 2018 we estimate 2,700 fare-dodgers received the wrong penalty after not paying for their journey. The average penalty was around £4.40 extra per person.

"We are reviewing all penalty fares issued in this period to arrange refunds wherever possible."

The RDG said it has conducted an investigation and will ensure train staff are properly trained.

Around half of people affected will be issued refunds for the amount they were overcharged, but in many cases the train companies do not have fare dodgers' full contact details.

Posters will be displayed at stations in the coming weeks to advise anyone who thinks they may be owed money to get in touch with the relevant operator.

An RDG spokesman said: "When people haven't paid to travel, it's important for train companies to take a firm but fair approach because fare dodging denies the railway around £200 million a year which could otherwise be invested to improve services for all passengers.

"People who have been charged a penalty fare shouldn't be overcharged, though.

"We have investigated this issue and will ensure that staff have the right advice and people affected are reimbursed quickly and easily."

On routes where penalty fares apply, passengers must buy a ticket before boarding a train at stations where there is an open ticket office or a functioning ticket machine.

If a ticket inspector catches anyone who avoids doing this, they can issue them with a penalty fare notice.

Penalty fares are £20 or twice the appropriate single fare, whichever is higher.

The single fare was previously based on the price of a ticket valid at any time of day.

But since April last year the price of off-peak fares should be applied if the tickets were available for the journey taken.

Thameslink has been approached.