Only three-quarters of first class mail delivered on time

Just over three-quarters of first class letters, or 75.7%, were delivered on time by Royal Mail in the year to the end of March, far off its target of 93%.

The latest quality-of-service report reflects the postal firm's performance under its new private owner, Daniel Kretinsky's EP Group, whose takeover was approved by shareholders at the end of April last year.

Ofcom said it was "very concerned" by the figures. The BBC understands the regulator expects to open a probe into Royal Mail's performance early next week.

Royal Mail said its service was improving and that it was on track to hit new reduced targets – of 90% for first class delivery and 95% for second class – by this time next year.

Chief operating officer Jamie Stephenson said: "We're putting significant investment into improving reliability and reaching these new delivery targets, but delivering lasting change across a network of this scale takes time."

The firm said it was investing £500m over the next five years as part of its improvement plan.

The postal service has faced years of criticism from politicians and the public over the slowness of its letter delivery.

The annual figures published on Friday show performance worsened compared to the previous year when the company was still listed on the London stock market.