Britain's Royal Mail said on Wednesday that it was experiencing a severe disruption to its export services after “a cyber incident”.
The service said it was “working with external experts” and was temporarily unable to send items including letters and parcels out of the country.
The National Cyber Security Centre was working with Royal Mail and the National Crime Agency to understand the extent of the problem.
“We have asked customers temporarily to stop submitting any export items into the network while we work hard to resolve the issue,” Royal Mail 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.
“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.”
It was not immediately clear how long it would take to fix the issue.
The National Crime Agency has confirmed it is also looking into the incident.