Royal Mail is experiencing “severe service disruption” to its international export services following a cyber incident.
The company said it was temporarily unable to despatch export items including letters and parcels to overseas destinations. It did not immediately give any more details on what the cyber incident entailed.
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.
“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.”
Separately on Wednesday, the company rejected claims it is planning to sack thousands of workers, insisting such statements are “designed to mislead and create fear and uncertainty" among their employees.
The company's comments come after MPs debated the future of postal services, with Labour's Rachael Maskell comparing Royal Mail to P&O Ferries and criticising the Government for standing by as the "next chapter of fire and hire plays out".
Members of the Communication Workers Union (CWU) staged a series of strikes last year in a bitter row over jobs, pay and conditions but fresh talks to resolve the long-running dispute started on Monday and will continue until January 20.
A Royal Mail spokesperson said: “We have written to the CWU on multiple occasions to correct the false allegations that Royal Mail is planning to 'sack' thousands of workers and wants to become 'another courier company'.
“This is simply not true. The statements are designed to mislead and create fear and uncertainty amongst our employees."