Severe disruption to Royal Mail’s overseas deliveries has been caused by ransomware linked to Russian criminals, the BBC has been told.

The cyber-attack has affected the computer systems Royal Mail uses to dispatch deliveries abroad. Royal Mail has been warning customers since January 11 of disruption due to a “cyber-incident.” Its latest advice is for people not to try to send international letters and parcels until the issue is resolved.

Ransomware is malicious computer software that encrypts data and locks up systems. The ransomware used in the attack is “Lockbit”, according to a source close to the investigation. Computer security firms say the software has been developed and used by criminal gangs with links to Russia.

Ransomware attacks are a persistent threat to organizations around the world, with attacks happening on a near-daily basis.

But this situation is highly significant, as Royal Mail is what is deemed “critical national infrastructure” — that is, it is critical to the U.K. economy.