Royal Mail says it can't send letters or parcels overseas because of disruption due to what it describes as a "cyber incident".
"We are temporarily unable to despatch items to overseas destinations. Some customers may experience delay or disruption to items already shipped for export," the company said in a statement.
"We would like to sincerely apologise to impacted customers for any disruption this incident may be causing," Royal Mail added.
The incident has also resulted in minor delays to letters and parcels being sent from overseas -- but domestic deliveries within the UK are unaffected.
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The company has asked people to stop attempting to post items for delivery abroad while it investigates what happened.
"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 said.
"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."
A statement by the Information Commissioner's Office said: "The Royal Mail has made us aware of an incident and we will be making enquiries."
Exactly what sort of cyber incident has occurred, how it happened or how long it will cause disruption for is yet to be disclosed.
NCSC, the UK's cybersecurity agency, 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."