Royal Mail - which is owned by FTSE 100-listed International Distributions Services PLC (LSE:IDS) - said it is unable to send letters and parcels internationally due to a cyber incident.
The company is asking customers to refrain from sending items abroad while they work to resolve the issue. There have also been minor delays to deliveries within the UK, but domestic deliveries have not been affected.
Royal Mail apologised for the disruption and said that teams are working around the clock to investigate the incident and will update customers with more information as it becomes available.
In a statement on its website, the company referred to a "cyber incident" instead of an attack, with the cause of the issue currently unknown.
Royal Mail said it has notified regulators and relevant security authorities, had called in cybersecurity experts and was “working round the clock to resolve this disruption”.
“We would like to sincerely apologise to impacted customers,” Royal Mail said.