Royal Mail in the United Kingdom says it is facing severe disruption to its international export services following what it describes as "a cyber incident".
"We are temporarily unable to despatch items to overseas destinations," Royal Mail, one of the world's largest post and parcel firms, said in a service update on its website.
It advised customers to temporarily hold any export mail items while it works to resolve the issue.
The company says thousands of businesses use it to export around the world.
Royal Mail, part of International Distributions Services Plc, said it was working with external experts to investigate the incident and had also reported it to regulators and security authorities.
The UK's data protection watchdog, the Information Commissioner's Office, said it would be making inquiries while the National Cyber Security Centre said it was working with the company alongside the National Crime Agency to "fully understand the impact".
The 507-year-old Royal Mail, which was privatised in 2013, had a rocky year last year, losing millions of pounds due to lower mail volumes and hit by a series of staff strikes in a long-running row over pay and conditions.
The Communication Workers Union, which represents more than 115,000 postal workers at Royal Mail, is planning further industrial action in the ongoing dispute, with a fresh ballot due to open later this month.
The "cyber incident" is the latest in a growing list of high-profile cybersecurity events in the UK.
The Twitter accounts of at least two senior government ministers appeared to have been hacked in recent weeks, while the Guardian newspaper said last month it had been hit by a "serious IT incident" believed to be a ransomware attack.
Royal Mail said its import services remained operational, albeit with minor delays.