As many as 10,000 rail travellers have been overcharged after being caught and penalised for not having a valid ticket.

The Rail Delivery Group (RDG), the industry body that represents private train operators as well as Network Rail, has admitted that tickets inspectors have overcharged fare-dodgers to the tune of tens of thousands of pounds after rail companies failed to implement a rule change introduced in April 2018.

The RDG said eight rail firms had failed to implement the new rules, which reduced the financial punishment for some travellers caught ticketless depending on the time of their journey.

Up to 10,000 passengers could have been affected by the mistake – which was discovered only last month – through overcharging by Chiltern Railways, Govia Thameslink Railway, Great Western Railway, Greater Anglia, Northern, Southeastern, South Western Railway and London Northwestern.

“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 £200m a year, which could otherwise be invested to improve services for all passengers,” a spokesman for RDG said. “People who have been charged a penalty fare shouldn’t be overcharged, though.”

RDG said that about half of the people affected would be issued refunds for the amount they were overcharged but in many cases the train companies do not have the full contact details of those penalised.

About 1,500 Southeastern passengers were affected by an average of £8. London Northwestern Railway estimates it overcharged 2,700 people a total of £12,000, or £4.44 per person on average. Great Western Railway said it would be posting cheques to passengers for whom it has details, with the average refund £6 per person, according to PA Media.

“We have investigated this issue and will ensure that staff have the right advice and people affected are reimbursed quickly and easily,” the RDG spokesman said.

Posters will go up in stations over the coming weeks highlighting the situation and asking anyone who thinks they may have been overcharged to contact their train company.

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.

These are also given to passengers unable to produce a railcard after buying a discounted ticket, travelling in first class with a standard ticket or staying on a train beyond the destination paid for.

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.

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