Long queues are predicted at post offices on Monday as branches struggle to implement radical changes to mail charges, a consumer watchdog warns.
A Postwatch survey of 307 branches early in August found a third were not ready for the changes, which will price mail by size as well as weight.
A third had not put up posters about the change and 34% had no template to check the size of post, Postwatch said.
Royal Mail said staff had been trained in the changes.
However, Postwatch predicted long queues as staff and customers deal with the changes at the UK's 14,400 post offices.
The survey also found there were no leaflets about the changes at 32% of offices.
Basic information
Under the Pricing in Proportion (PiP) scheme, the price of an average first class letter will be 32p.
Anything that is longer than 240mm, wider than 165mm or thicker than 5mm will be classed as a large letter and cost 44p if less than 100g.
"Basic sources of information such as posters and leaflets should have been available at all post offices well in advance," Judith Donovan, the chairwoman of Postwatch's Trade Association Forum, told the Daily Telegraph.
"A third of post offices failing to do the bare minimum points to a potentially complacent approach in introducing what will be the biggest change to our postal service since the Penny Black.
"While we understand that in the immediate post-introduction period Royal Mail will be flexible in terms of not surcharging customers who have underpaid postage, the best way of ensuring that customers are paying the correct amount is by making every effort to alert them to the change and its implications."
A Royal Mail spokesman said the survey was conducted before posters and other information had been fully distributed.
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