Royal Mail on Tuesday temporarily stopped access to its online postage and parcel tracking service after reports some customers were able to see information on other users' orders, the British company said.
The issue comes a day after Royal Mail's largest labour union rejected a new pay offer in a long-running dispute, and as the firm works on a plan to shift its focus to parcels and improve efficiencies amid sliding letter volumes.
The problem seems to have started around 1300 GMT, according to a website tracking the developments, but Royal Mail, owned by International Distributions Services, did not give details of how many customers' data may have been compromised nor how long it might take to bring the website back online.
"We fully understand and apologise for the inconvenience caused by this. Our engineers are working as hard as possible to get the site back up and running as expected," it said, adding the company was treating a fix as "the highest priority".
Royal Mail's "click and drop" service allows customers to pay for postage online, print labels, and track parcels and posts once they have been dropped off, and is used by individuals and businesses.
Sky News first reported the incident on Tuesday.
(Except for the headline, this story has not been edited by NDTV staff and is published from a syndicated feed.)
