A train company has apologised after some customers who made contactless payments were undercharged for up to four months.

London Northwestern Railway passengers were left confused after noticing the cost of their daily commute suddenly increased by as much as £13.50.

A technical issue meant some customers using contactless cards had been undercharged for some journeys since February 2025 but since Sunday they had been paying the correct fares.

A London Northwestern Railway spokesperson said: "We apologise for any inconvenience this has caused."

Public relations worker Harriet, who did not want her surname revealed, told the BBC her £22.50 commute to the capital from Berkhamsted was now £36.

"I'm massively worried about it… it really adds up," the 29-year-old said.

Other concerned commuters took to social media to question why their fares had increased.

London Northwestern Railway said the issue could have affected journeys within its contactless zone, which included stations between London Euston and Bletchley, and its branch line to St Albans Abbey.

Passengers could check the price of a contactless pay-as-you-go journey using the Transport for London website, it added.

Regulated rail fares in England and Wales rose by 4.6% in March.