Royal Mail has been warned by its regulator it must improve its service as delivery delays left some receiving Christmas post in the middle of January.
As the impacts of the Covid pandemic subside, the postal service must improve, watchdog Ofcom said.
It added action would be taken if the Royal Mail failed to meet its annual targets.
Royal Mail has said delivery offices have been struggling due to staff absences related to the Omicron wave and “other local factors”.
Postal workers say they are facing intense pressure, with a hugely increased workload and more than 900,000 Covid test kits to deliver every day.
In some offices a third of employees have been off sick during the Omicron wave, a Royal Mail spokesperson said.
People have reported waiting weeks for post to be delivered across the country – with delays said to be especially bad in parts of London, Manchester, Hertfordshire, Scotland and Wales.
Of the 56 worst-hit areas, 18 are in the capital. In southwest London, Christmas cards are coming through letterboxes weeks after the holidays.
One disgruntled resident branded the postal service a “disgraceful rip-off” on Friday, complaining a card with a first-class stamp arrived more than five weeks after it was sent.
Another offered a tongue-in-cheek thank you to Royal Mail for “making Christmas last all year” after receiving cards weeks late.
Others across the country say they have not got hold of crucial letters on time, including details of medical appointments, bills and parking fines.
Sarah Jones, MP for Croydon Central, said she had asked Royal Mail “to investigate the current state of affairs”.
In a Facebook post she said: “It is my understanding that both the Croydon and South Croydon Delivery Offices are currently unable to complete all deliveries six days a week due to absences.”
She added postal workers had been “invaluable” throughout the pandemic and said they deserve “our utmost respect and gratitude”.
The Independent heard from one postman who said he’s been suffering from anxiety as the pressure builds up.
After months of intense work, he said he only realised how much stress he’d been under when his leg began shaking uncontrollably during a visit to the doctor.
According to Royal Mail, absence rates are currently double what they were before the pandemic. There were 15,000 staff off work two weeks ago. Those numbers are now beginning to come down, with 2,000 employees back at work last week.
Royal Mail said deliveries are “operating as normal across most of the country”.
A spokesman said: “We aim to deliver to all addresses we have mail for, six days a week.
“In a small number of local offices this may temporarily not be possible due to local issues such as Covid-related self-isolation, higher than usual levels of sickness absence, resourcing or other local factors.
“We are providing targeted support to the local offices affected by these issues. Our staff are continuing to work incredibly hard, as they have done throughout the pandemic, and we are thankful for all of their efforts and determination.”
Ofcom requires Royal Mail to deliver at least 93 per cent of first-class post across the UK within one working day of collection, and 98.5 per cent of second-class post within three working days.
But at the start of the pandemic, it acknowledged the pressures the postal service was under and gave Royal Mail time to readjust. But this period ended in August last year.
It said Royal Mail is exempt from its targets every December because of the volume of pre-Christmas deliveries.
A spokesperson for Ofcom said: “We know how important a reliable postal service is to customers, and we can take action if Royal Mail fails to meet our annual targets.
“We closely monitor its delivery performance throughout the year, and have made it clear to the company that it must improve as the impacts of the pandemic subside.
“We will assess Royal Mail’s compliance after it has reported on its overall performance for the whole financial year.”