Royal Mail has said it suffered a cyber incident that caused “severe service disruption” to its international export services.
A statement said it was temporarily unable to despatch export items, including letters and parcels to overseas destinations.
People have been told to avoid posting any items abroad while the problem is tackled.
Royal Mail said: "We have asked customers temporarily to stop submitting any export items into the network while we work hard to resolve the issue.
"Some customers may experience delay or disruption to items already shipped for export.
"Our import operations continue to perform a full service with some minor delays.
"Our teams are working around the clock to resolve this disruption and we will update customers as soon as we have more information."
Royal Mail has said that it has reported the matter to "security authorities".
The statement continued: "We immediately launched an investigation into the incident and we are working with external experts.
"We have reported the incident to our regulators and the relevant security authorities.
"We would like to sincerely apologise to impacted customers for any disruption this incident may be causing."
A National Cyber Security Centre spokesman said: "We are aware of an incident affecting Royal Mail Group Ltd and are working with the company, alongside the National Crime Agency, to fully understand the impact."
The news delays comes after weeks of disruption in the UK due to Royal Mail strikes.
Royal Mail has been locked for months in an increasingly bitter battle with the Communication Workers Union (CWU) over pay and changes to working conditions.
Last month, Royal Mail said it wants customer services to return to normal "as quickly as possible".
"After industrial action takes place, we'll be increasing our network capacity and using additional resources to assist with getting services back to normal," it said. "Deliveries will also resume the day after strike action finishes."
