Royal Mail narrowly missed its target for delivering more than nine out of 10 first class letters a day after posting, in the run up to Christmas, its busiest time of the year, new figures showed.
They said 92.3% of first class mail arrived on time in the final three months of the year, 0.7% below the target.
The second class target of 98.5% of letters arriving within three working days of being posted was met.
Royal Mail said the figures showed that the overwhelming majority of the average of 59 million letters, packets and parcels handled every day, arrived on time.
Mark Higson, Royal Mail's managing director for operations and modernisation, said: "Royal Mail's target for next-day delivery of first class mail is challenging and we are disappointed on behalf of our customers to have narrowly missed hitting it in the autumn. Our postmen and women remain determined to deliver the best possible service to our customers."
Royal Mail said the scale of changes it was bringing in, including modernisation and streamlining of deliveries, led to a "temporary adverse impact" in some areas.
The highest performing postcode areas included Edinburgh, Harrow and Uxbridge, both Middlesex, Ipswich, Inverness, Blackburn and Burnley.
Robert Hammond of Consumer Focus said: "Consumers expect a good level of service from Royal Mail, and delivery targets are set as a benchmark to make sure they receive this. The overall results are narrowly down on the last quarter, and show a patchy picture, with little discernible or consistent upward trend.
"People have seen stamp prices rise over recent years. With the prospect of more price rises to come, consumers will expect a steady improvement in delivery performance as a result.
"While consumers might see a dip in performance where modernisation is happening, this shouldn't be for an extended period as appears to be the case in some areas. Some post code areas have been failing for a number of quarters in succession and this is an issue which Royal Mail and the regulator Ofcom need to keep a close eye on."