The region’s rail company is one of eight from around the country which charged penalty fares based on the full-fare even for people who were travelling on an off-peak train.

Industry body the Rail Delivery Group said the companies punished ticketless travellers too harshly by not implementing a rule change made in April last year. It has conducted an investigation and will ensure train staff are properly trained.

A spokeswoman for Greater Anglia said the company would be compensating about 600 passengers who would be entitled to a refund of between £2 and £6.

She said: "We have their details and we will be contacting them pro-actively. If people want to claim a refund they can, but we will be writing to them about this."

A passenger watchdog demanded that train operators must do "everything possible" to prevent a repeat of the error.

David Sidebottom, director of watchdog Transport Focus, said: "Passengers will want urgent answers as to why this error occurred and assurances from the rail industry that it is being investigated and fixed immediately.

"Train companies need to do everything possible to demonstrate that they have a clear plan to get things back on track, provide train staff with the correct advice to avoid this happening again and make sure passengers aren't out of pocket."

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 handed out 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.

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