Royal Mail PLC (LSE:RMG) is struggling to make Christmas deliveries at some of its offices as absence levels have more than doubled since before the Coronavirus (COVID-19) pandemic.
The company said 21 offices are currently listed as experiencing delays out of 1,200 delivery offices around the country.
It blamed disruption to services on “Covid-related self-isolation, high levels of sick absence, resourcing, or other local factors”.
Royal Mail said most of the country is receiving a normal service.
But a source with close knowledge of its operations told the BBC that the internal situation is "horrific" due to sickness and increased demand in the run-up to Christmas.
"It's much worse than a normal Christmas," the source said.
"The combination of people off with stress or Covid, combined with increased demand to send goods by post and the build-up to Christmas, had led us here," they said.
"Sickness levels are around twice the normal levels," they added.
A Royal Mail spokesperson told the BBC: "Every item of mail is important to us. Where offices are experiencing local challenges, such as higher than usual absence levels, we have asked teams to rotate deliveries to minimise delays for individual customers.
"We have also reminded colleagues that the delivery, collection and processing of letters and parcels should be treated with equal importance."
Royal Mail already has the largest fleet of vans in the country, including 40,000 of its distinctive red vehicles.
The company is attempting to revise hundreds of routes to make its operations more efficient and workloads fairer, the Mail on Sunday reported.
However, one source told the newspaper: “The revisions are delaying the post. It's a 'computer says no' scenario. The technology says the routes can be done but it doesn't take into account roadworks, traffic jams and blocks of flats with 30 addresses the posties have to get to.”
Senior sources denied the revisions are the cause of the delays.