An Post's mail delivery rates are falling significantly behind their own targets, set seven years ago, according to a study by Ipsos MRBI.
A tracking survey found 85% of standard letters, large envelopes and packets reached destinations within one day.
But ComReg warned it fell far short of An Post's target of 94% one-day delivery.
The assessment also ruled out delays in the service brought about by two spells of freezing weather at the end of November into December last year.
The best-performing postal routes are locally outside Co Dublin, with mail sent into the capital getting the worst score, of 83% for next-day delivery.
The study also found that 99% of all mail was delivered within three working days, narrowly failing to hit targets by just half a per cent.
The report was based on 28,487 test mail items, posted and delivered to identify on time reliability.
An Post made a €5.8m profit last year and has warned it will need to reduce its workforce by 1,600 by 2015 on top of the 730 lay-offs over the past two years.