The UK postal service company said it was experiencing "severe disruption" to its international export services and is temporarily unable to dispatch items overseas. It has yet to provide an update on when the incident is likely to be resolved and shipping would resume.
It is believed to have already left more than half a million letters and parcels stuck in limbo, according to reports at the end of last week.
The attack is suspected to have come from a Russian-linked ransomware gang called Lockbit, as reported first by the Telegraph.
A Royal Mail distribution centre in Northern Ireland revealed its printers began "spurting" out copies of a ransom note on Tuesday, saying "your data are stolen and encrypted."
Lockbit, which is believed to have close links to Russia, was also behind a major hack of car dealership Pendragon last year, which refused to pay a ransom payment of $60m.
Royal Mail would not comment on the hacking reports, but said it had launched an investigation into the incident and had reported it to its regulators and security authorities. The firm said today (Monday) that it wants to avoid a build-up of items to be sent overseas left sitting in its sorting offices.
In a statement, it 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 despatched may be subject to delays."
The company has been hit by disruption in recent months, with postal workers staging multiple walkouts in December 2022 in a long-running dispute over jobs, pay, pensions and conditions.
The ongoing saga has caused havoc for businesses who rely on the delivery services, with major retailers such as Moonpig, Card Factory and Asos partially blaming the strikes for a drop in sales towards the end of last year.
Royal Mail has also suffered heavy losses from the crippling industrial action. In November, it said that three days of strikes in the first half of the year had cost it around £70m, while a further five days of strikes in October cost it another £30m.
More strike days in November and December, including across the crucial Black Friday discounted shopping days, as well as the temporary suspension of international deliveries, are likely to have struck further big blows to the loss-making delivery firm.
The company recently rubbished reports that it was planning to sack thousands of workers in order to save money, despite also targeting £350m in cost efficiencies.
Instead, it confirmed that it would be reducing 10,000 full-time equivalent roles, which would be achieved through "natural attrition, reducing temporary workers and a generous voluntary redundancy scheme which has been oversubscribed".
