Royal Mail has said it is facing severe disruption to its international export services after a "cyber incident".
The company is temporarily advising customers to hold any export mail while it tries to resolve the issue.
Royal Mail added that it had launched an investigation into the incident, was "working with external experts" and "sincerely apologised" to customers for the disruption.
Its import services remain operational, but with minor delays.
"We are temporarily unable to despatch items to overseas destinations," the firm said.
"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."
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 National Crime Agency confirmed it is also looking into the incident.
The incident comes at a difficult time for Royal Mail, which is currently embroiled in a bitter row over jobs, pay and conditions with the Communication Workers Union that resulted in a series of strikes last year.