An Post's mail delivery rates are falling significantly behind targets the company set seven years ago.
A tracking survey of postal service and quality has found 85% of standard letters, large envelopes and packets reached destinations within one day.
But the Commission for Communications Regulation (ComReg) warned that it still fell far short of An Post's own target for 94% one day delivery.
The scorecard showed the service managed a 1% improvement compared to 2009.
ComReg said: "Despite the An Post performance standard of 94% being in place since 2004, a significant deficit still prevails between actual performance and An Post's performance standard."
The assessment, the eighth independent report on An Post's quality, ruled out delays in the service brought about by two spells of freezing weather at the end of November into December and the fortnight leading up to the new year.
The best-performing postal routes are locally outside County Dublin where there is an 86% next day success rate. Mail sent from outside the county into the capital had the worst score of 83% for next day delivery.
The study, conducted by Ipsos MRBI, found that 99% of all mail was delivered within three working days, narrowly failing to hit targets by just half a per cent.
An Post said it has made sustained improvements since the targets were set in 2004 with the next-day delivery scores in 2006 at just 72% before climbing to 84% by 2009.
"What we have been aiming for is sustained improvements. The most important thing for An Post is that we stick with it and improve," a spokeswoman said.