On the ‘International Incident Bulletin’ on its website, the postal operator stated: “We are temporarily unable to despatch items to overseas destinations.

“We strongly recommend that you temporarily hold any export mail items while we work to resolve the issue. Items that have already been despatched may be subject to delays. We would like to sincerely apologise to impacted customers for any disruption this incident is causing.

“Our import operations continue to perform a full service, with some minor delays. Parcelforce Worldwide export services are still operating to all international destinations though customers should expect delays of one to two days.

The postal operator said its teams were working "around the clock" to resolve the disruption and it would update customers "as soon as we have more information".

“We immediately launched an investigation into the incident and we are working with external experts. We have reported the incident to our regulators and the relevant security authorities.”

The incident will cause further disruption for those reliant on Royal Mail’s services, following months of strikes involving over 115,000 postal workers across the Royal Mail network.

Former Royal Mail CEO Rico Back criticised the current management’s handling of the industrial dispute with CWU workers earlier this month in an interview with the Sunday Times.