Confusion as 1 in 8 letters and parcels now has incorrect postage

The watchdog Postwatch slammed Royal Mail for a record which it said is 'indefensible and unacceptable.'

To test the new 'Pricing in Proportion' system, Postwatch sent 1,080 items from 360 Post Offices around Britain.

The correct first class postage for each item should have been 44p - but customers were charged anything between 32p and £1.27.

At each Post Office, Postwatch investigators went up to the counter and asked a member of staff: "How much would it cost to send this item?" Despite asking a Post Office worker for advice, the wrong price was given for 132 of the items.

The items were an A4 envelope containing one sheet of paper, an A5 birthday card with a badge attached, and a CD in a packet.

Millie Banerjee, chairman of Postwatch, said: "One in every eight customers being incorrectly charged is indefensible and unacceptable".

She is calling for extra staff training to stop chaos at Christmas when the number of letters and cards sent through the post soars.

On a typical day, about 80 million items are sent through the post, but an extra 750 million, mostly Christmas cards, are sent in December.

Postwatch fears the Christmas rush will push the new pricing system to breaking point.

Introduced on 21 August, Pricing in Proportion was the end of a simple system which charged a fixed amount for everything up to 60g.

For the first time, people sending anything through the post have to think about the size and thickness, not just the weight.

According to Postwatch, the majority were under-charged, not over-charged. Of the 132 incorrectly charged items, 15 were over-charged but 117 were under-charged.

Under-charging can be a real hassle - and an expensive one too, particularly when you discover you did not even want the item that was sent to you.

If you receive an under-charged letter, you have to go to the local collection office, pay a £1 administration fee plus the cost of the under-paid postage.

In the majority of cases, Royal Mail said yesterday it is simply putting a sticker on the letter which says: 'This item has been underpaid.' It adds: 'New postal rates were introduced on 21st August 2006. On this occasion, you have nnot been surcharged but in future you may be. Please contact the sender.'

This 'amnesty' has been in place since the summer, and Royal Mail will not say how long it will continue.

A Royal Mail spokesman said: "Royal Mail's huge effort to raise awareness about its new pricing system has been successful, as Postwatch has said. 'But their claim about the number of people being charged incorrect postage is totally wrong."

Postwatch only tested 'unusual' items, such as sending a CD, but the vast majority of mail is a standard letter size, and the cost has not changed.

For stamped mail, over 85 per cent is either the same price or cheaper under the new system. A new advertising campaign in regional and national newspapers to remind before they send their Christmas cards is planned.

It follows a £10 million TV and newspaper advertising campaign over the summer to explain the changes to millions of people.

More than 80 per cent of people are now aware of Pricing in Proportion, according to the Postwatch survey.