The postal company, part of International Distributions Services Plc, said it had also resumed exports of letters and from 1900 GMT customers could begin submitting international export letters to its network.

However it said that while it trials the workarounds, it was continuing to ask customers not to submit new export parcels. Import operations and domestic services remain unaffected.

“Our initial focus will be to clear mail that has already been processed and is waiting to be despatched,” Royal Mail said in a statement.

Royal Mail, one of the world’s largest post and parcel firms, said it was still working with external experts, security authorities and regulators to mitigate the impact of the incident.

The Telegraph newspaper reported last week that Lockbit, a ransomware group that some cybersecurity experts say has members in Russia, was behind the incident. Royal Mail declined to comment on the report.

The number of ransomware incidents – where hackers encrypt victims’ data with software and offer them a key in return for money – has risen sharply in recent years.