A migration of online data to new servers at Royal Mail has knocked out a series of its most important postage websites for consumers and businesses, including a Price Finder page, for nearly all of this week.
Royal Mail's online post redirection and redelivery services, for customers who missed deliveries when they were out, have also been intermittently affected, alongside online stamp sales, the BBC reported.
The server migration is a major step for Royal Mail, which has been working on the move for 18 months.
The post service told customers on its website of the "technical difficulties" affecting the sites. The Price Finder site details postage costs based on information input by the sender, and allows people to buy print their stamps instead of queuing in Post Office branches.
On the Price Finder site, Royal Mail posted a message saying: "We are sorry for any inconvenience this is causing. We are working hard to fix these problems as quickly as possible."
Royal Mail said the problems would be solved "very soon".