Royal Mail's Christmas postal service is anything but first class, an independent survey has found.

Around half of all first class mail arrived late during the festive period, according to a study by a consumer watchdog Postwatch Scotland.

It studied more than 16,000 items received between December 1 and 24, 2007.

They found that, overall in Scotland, just 51% of first class mail was received the next day after posting. The worst area was Paisley where only one in three items stamped first class arrived on time.

Nationwide, only 61% of items sent by Special Delivery arrived the next working day.

The best value for money option was second class, but even then, only 83% of second class mail was received within three working days.

Over 25% of all mail received had no date on the envelope at all, so customers could not tell whether or not it had been delayed.

Alan Alexander, chair of Postwatch Scotland, said: "We have been advising customers for a number of years that Royal Mail's service which represents the best value for money during December is second class post.

"Our study clearly illustrates why. First class post drops sharply in the build-up to Christmas - exactly when many customers want a reliable, timely service. A significantly higher proportion of second class mail arrives on time. For 10p less per stamp, that is a much better deal."

The Postwatch study involved 217 volunteers who recorded 16,433 items of mail received. Performance in each of 16 postcode districts varied, with Paisley being the worst area for both first and second class mail. Dumfries and Galloway had the best performance for second class post with 93% arriving on time. Kirkcaldy had the best performance for first class mail - 59% arrived on time.

Mr Alexander said: "We are calling for Royal Mail to take action to improve the quality of its services during the Christmas period. While we recognise the significant increase in mail at this time, customers are entitled to get the service they are paying for.

"We are calling for all mail operators, including Royal Mail, to include a date of posting on every envelope, so that customers will know the quality of service they are receiving."

Royal Mail in Scotland delivers to 2.5 million addresses every day and 250 million items of mail at Christmas.

A Royal Mail spokeswoman said: "Postwatch's sample of mail and those taking part in this survey is tiny compared to the amount of mail we deliver at Christmas and is not representative of the fantastic effort that our people make.

"The sample only looks at a 300 people across the country. It is a well-known fact that targets over the Christmas period are suspended by the regulator as it recognised that it is an exceptionally busy time as Royal Mail deals with around two billion items of mail.

"Royal Mail is committed to deliver the mail as quickly as possible but it must be remembered that there can be reasons why mail can be delayed, such as bad weather."

She added: "Thirty-six pence for a first class stamp is still excellent value for money and our own independently monitored figures show that quality of service across the UK for first class mail stands at 79.2% after a period of strike action."