Cyber security experts warned that the damage to Royal Mail’s computer system looked extensive.
Jake Moore, an expert at software business ESET, said: “After multiple strikes and labour shortages, this attack will have come at one of the worst possible times for the postal service.
“The attack seems to have severely impacted their systems, and it could take weeks for things to return to the way they were before the incident.”
Royal Mail customers had already been experiencing delays following a wave of strikes that left the postal service grappling with huge backlogs.
Some customers were already complaining that it was taking weeks for their tracked parcels to be dispatched from the Heathrow sorting centre.
Sheila Valecha, said she sent an international letter to India on December 9 but had received no updates after it was “bagged” at the facility.
Meanwhile, another customer said an item they had posted more than 40 days ago was still at the centre and another said that they had posted two packages to Thailand before the strike action – with both still at Heathrow .
“Will it ever leave the UK this year?,” he asked Royal Mail on Twitter.
Customers whose packages have been tracked to Langley told the Telegraph they had not been contacted by Royal Mail directly about the cyber incident.
Although Royal Mail delivers domestic post for the British Forces Post Office, mail to Ministry of Defence personnel overseas is not thought to be affected as it is taken directly by military plane to bases abroad.