The postal regulator has launched an investigation into Royal Mail for once again missing its annual delivery targets, with almost a quarter of first-class mail arriving late.

The company, which has been fined £37m since 2023 for routinely failing to meet delivery targets set by Ofcom, revealed on Friday that 24.3% of first-class mail failed to arrive on time in the year to the end of March.

The figures show that Royal Mail’s performance has worsened compared with the previous year, when 23.5% of first-class mail failed to arrive within the one-working-day target set by the regulator.

Under the watchdog’s rules, 93% of first-class mail must be delivered within one working day of collection, excluding Christmas.

The latest delivery figures published by Royal Mail showed that it managed to deliver 90.2% of second-class mail within the three-working-day limit, well short of Ofcom’s target of 98.5%.

In October, Ofcom fined Royal Mail £21m for missing annual delivery targets, the third-biggest financial penalty it has ever issued to any company.

On Monday, Ofcom said that as part of its investigation it will look at whether Royal Mail is prioritising parcel delivery over letters, a practice which whistleblowers and unions have alleged but the company denies.

“In deciding whether Royal Mail breached its regulatory obligations, we will consider all relevant factors,” Ofcom said. “This will include the question of parcel prioritisation, as well as identifying whether there were any exceptional events beyond the company’s control that may have affected its performance. Where we determine a breach of Royal Mail’s obligations during our investigation, we will consider whether to impose a financial penalty.”

Royal Mail has not hit its Ofcom-mandated delivery target for first-class post since 2017, or for second-class mail since 2020.

Last year, the Czech billionaire Daniel Křetínský completed a drawn-out £3.6bn purchase of Royal Mail’s owner, International Distribution Services (IDS), after the takeover was approved by a UK government national security review.

In April, Royal Mail faced a barrage of criticism as it raised the price of a first-class stamp by 10p, or 6%, to £1.80. The cost of a first-class stamp has more than doubled since 2020.

The cost of the second-class service rose by 4p, or 5%, to 91p. Royal Mail blamed the need for price increases on the “continued rise in the cost of delivery for every letter”.

Last July, Ofcom gave IDS permission to loosen Royal Mail’s universal service obligation (USO), ending second-class post on Saturdays and reducing the service to alternating weekdays from Monday to Friday.

A decade ago Royal Mail was delivering 20bn letters a year, but that has shrunk to 6.7bn and could drop to 4bn within four years. Over the same timeframe the number of addresses it serves has risen by 4m.

A spokesperson for Royal Mail said it would “engage fully with Ofcom”, that improving its quality of service was “a top priority” and that it was delivering a significant programme of change, backed by £500m of investment over five years.

“These reforms are designed to deliver long-term quality improvements for customers as we modernise the postal service and deploy the new delivery model, enabled by the changes to the universal service regulations that Ofcom introduced in July 2025,” they added.