Royal Mail said it had asked customers to stop sending international mail until the problem is resolved.
An online gang is behind the "cyber incident" disrupting UK Royal Mail's international delivery services, according to a ransom note seen by the Daily Telegraph on Thursday.
The British postal service said Wednesday it was suffering "severe disruption" to its international letter and parcel services due to the incident but did not explain its nature.
Royal Mail said it had asked customers to stop sending international mail until the problem is resolved. Items already shipped "may experience delay or disruption", it warned.
A note seen by the Telegraph identifies the culprits as ransomware gang LockBit.
"LockBit Black Ransomware. Your data are stolen and encrypted. You can contact us and decrypt one file for free," says the note, according to the newspaper.
The Russian-speaking hacking group is known for scrambling files on a host's computer and flashing up messages demanding cryptocurrency payment to resolve the issue.
Royal Mail declined to comment on the report.
LockBit has previously carried out attacks against critical infrastructure, hospitals and large industrial groups around the world, with ransom demands ranging from €5 million to €70 million ($5.1 mln to $71.4 mln).
It first appeared in January 2020, has been deployed against as many as 1,000 victims in the United States and other countries.
LockBit members have extracted tens of millions of dollars in ransom payments from their victims.
Royal Mail last November recorded a drop in revenue of more than 10 percent, blaming a drop in the volume of parcels -- which had been boosted by the pandemic -- and an ongoing industrial dispute over wages.
The company said in October it was considering cutting up to 10,000 jobs by August, out of more than 137,000 employees.
Set up more than 500 years ago, Royal Mail has experienced some of its most turbulent times during the past decade, particularly following its controversial privatisation in 2013.
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