Postal watchdog Postwatch has urged people to send Christmas cards second class because of the "dramatic" decline in the delivery of first-class letters in recent years.
It said more than three out of every 10 first-class letters posted over the past two years at Christmas time had failed to arrive the next day.
The group said it was clear that a first-class stamp did not mean a first-class service at this time of year and recommended that people posted cards and letters second class.
Chairman Peter Carr said: "The well-known cliche is true - customers should post early for Christmas. 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 of December 18."
The Royal Mail accused Postwatch of giving customers the wrong advice.
A spokesman said: "First Class mail travels significantly faster than second class at Christmas - as it does at all other times of the year.
"If customers want their Christmas cards to travel faster then they should use first-class stamps. First-class mail gets the priority and is sorted before we handle second-class mail.
"Despite the huge volumes of mail at Christmas, the majority of first-class cards and all other first-class mail still arrives the day after posting."
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