Royal Mail customers should use second instead of first class postage this Christmas, as it is more reliable.
Three out of 10 items are sent first class
During the past two years, the first class service had missed more of its performance targets than the second class service, watchdog Postwatch said.
Overall, three out of 10 items posted first class failed to reach their recipient the next day, against a target of less than one in 10.
However, nine out of 10 second class items beat their three-day target.
Postwatch added that the efficiency of the first class post declined over Christmas as the Royal Mail faced delivering 2.1 bn items over the festive period.
"At Christmas time, a first class stamp does not mean a first class service," Peter Carr, Postwatch chairman said.
Royal Mail customers pay 28p for a first class stamp, 7p more than the price of a second class stamp, in the hope that the item will reach the recipient the next day.
Mr Carr said that poor performance meant that the first class premium wasn't worth it.
"Our review highlights that whilst the first class service falls to below 70% delivered next day, more than nine out of 10 second class items are delivered on time in the weeks before Christmas."
"Customers should therefore not only post early but also use second class stamps right up to the last recommended posting date, which is Saturday 18 December."
But in response, a Royal Mail spokesman told the BBC that any suggestion that the second class post was more efficient than the first was "completely incorrect".
"The first class mail is sorted in its entirety before the second class is even touched. "
"We aim to deliver as much of it as we can the following day. Even during the very busy Christmas period, the majority gets to its destination on time," the Royal Mail spokesman said.