Royal Mail has asked customers to not send items abroad until "further notice" following a cyber attack.
The delivery giant said it is continuing to experience "severe service disruption" following a "cyber incident" - however, the group has not provided further details on the issue.
In a statement, Royal Mail said: "To support faster recovery when our service is restored and to prevent a build-up of export items in our network, we're asking customers not to post international items until further notice.
"Items that have already been dispatched may be subject to delays."
The National Cyber Security Centre has said it's aware of the incident.
The group first announced it had been affected by a cyber attack last week.
The area affected by the cyber incident was the back office system which is used by Royal Mail to prepare mail for being sent abroad and to track and trace overseas items.
In its statement, Royal Mail said it was temporarily unable to despatch export items, including letters and parcels to overseas destinations.
Last week, people were told to "avoid" posting any items abroad while the problem is tackled.
When Royal Mail reported the news, it described the incident as a "cyber-incident" rather than an attack as the group did not know what had caused the problem.
It also said it had "immediately" launched an investigation into the incident and were working with its external partners.
However, it was later reported by news outlets that the incident had been an attack by a "ransomware gang" who reportedly have close links to Russia.
The Telegraph reported that printers at the Northern Irish Royal Mail distribution centre had begun "spurting" out copies of a ransom note.
The ransom note has been reported by the Telegraph as saying: ""Lockbit Black Ransomware. Your data are [sic] stolen and encrypted.
"You can contact us and decrypt one file for free."
The ransom group has also reportedly threatened to publish stolen data on the dark web.
Royal Mail declined to comment on this at the time of the report.
With Royal Mail asking customers to not send items using the overseas delivery service for the "foreseeable," it is unclear how long the disruption will go on.
Royal Mail is able to send items overseas to around 220 countries across the world, and in the year until March 2022, it sent around 152 million international parcels.
This equated to around 200,000 items a day and amounted to one-tenth of total parcel volumes.
