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.
Meanwhile, only 90.2% of second class letters were delivered within three working days against a target of 98.5%.
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.
It has been six years since the institution last met its letter delivery targets for second class post and ten years since it last met its letter delivery targets for first class post.
Its performance slumped during the Covid-19 pandemic and has failed to fully recover since.
In October last year, the regulator Ofcom fined Royal Mail £21m for missing the targets - the third-largest fine ever imposed by the communications watchdog.
Royal Mail was also fined in 2023 and 2024 for poor performance.
In February this year, postal workers told the BBC that some letters had been sitting undelivered for weeks and that they had been told to prioritise parcel delivery instead as it is more profitable.
Royal Mail executives were hauled in front of a parliamentary select committee in March to respond to the claims.
Kretinsky told MPs at the meeting that he was "deeply sorry for any letter that arrives late".
In response to the allegation that parcels were being prioritised, he said: "I have never heard any instruction or discussion, and have not participated in any exchange, that would sanction that Royal Mail is prioritising parcels over letters."
Reacting to Friday's performance figures, Citizens Advice policy director Tom MacInnes said poor performance at Royal Mail was "business as usual".
"What's worse, Royal Mail claims people will have to wait another year until it can meet its new, lower delivery targets," he added.
As part of its improvment plan, Royal Mail has given part-time postal workers the option to work longer hours.
It has also agreed a plan with Ofcom to scrap second class delivery on Saturdays as part of its new model.
Ofcom has also reduced Royal Mail's letter delivery targets. Since April this year, the service has been measured against a new lower target: 90% of first class letters must arrive by the next working day and 95% of second class letters must be delivered within three days.
Ofcom said that "maintaining the current targets – which are more stretching than comparable European countries – would carry higher costs which would need to be recovered through higher prices".
