Royal Mail has published a list of 28 areas in the UK suffering a limited delivery service because of the impact of coronavirus.
People in the areas, 27 in England and one in Northern Ireland, are no longer receiving regular post because Royal Mail staff are either off sick with COVID-19 or are self-isolating.
Of the areas listed, 13 are in or near London, while others include Leeds in West Yorkshire, Chelmsford in Essex, Widnes in Cheshire and Margate in Kent.
The area affected in Northern Ireland is Magherafelt in Co Londonderry.
Watch: The 28 UK areas suffering COVID postal delays
Some residents have complained their mail has not been delivered for more than a month.
Fears have also been expressed that elderly residents will not receive letters notifying them of how to access the coronavirus vaccine.
Seven new large-scale vaccination centres opened across England on Monday.
Watch: New vaccine centres open around England
Valerie Wallbank, from Hainault, north-east London, told the Ilford Recorder she had not received any mail since 12 December.
“It’s quite serious,” she said. “Some people are not bothered because they do everything online but for some of us it’s quite important.”
Wes Streeting, Labour MP for Ilford North, said the problems were having “a serious knock-on impact on people getting important NHS correspondence, both about their vaccination and also other routine appointments”.
He told the Daily Telegraph: “I have been told locally that Royal Mail are prioritising the delivery of parcels to get the backlog down, but I wonder if there is a way for NHS correspondence to be clearly labelled as such and prioritised for delivery.
“No one wants to see a situation where people are missing their vaccination appointments.”
When Streeting raised the issue on Twitter on Monday, vaccines minister Nadhim Zahawi said the government will work with Royal Mail to make sure people are notified of their vaccination appointments.
The issue has prompted calls, including from MPs and the Communication Workers Union (CWU), for postal workers to be added to the priority list of people receiving coronavirus vaccinations.
In a statement, Royal Mail said: “The combination of greatly increased uptake of online shopping, and the ongoing COVID-19 restrictions, mean that all delivery companies are experiencing exceptionally high volumes.
“Every single parcel and letter is important to us. Despite our best efforts and significant investment in extra resource, some customers may experience slightly longer delivery timescales than our usual service standards.
“This is due to the exceptionally high volumes we are seeing, exacerbated by the coronavirus-related measures we have put in place in local mail centres and delivery offices to keep our people and customers safe. In such cases, we always work hard to get back to providing our usual level of service as quickly as we can.”
It advised the people in the affected regions who have been left a “Something for you” card from the service to check its website to learn revised opening hours for when they can collect the item.
“If we’ve not left a ‘Something for you card’, and you do not have alternative arrangements in place, we’re sorry that your mail will not be available for collection,” the advice says.
Royal Mail’s 28 areas suffering limited delivery:
Barnsley (S70, S71, S72 and S75)
Bury St Edmunds (IP28-IP33)
Horsham (RH12, RH13)
Leeds City (LS1-LS5)
Magherafelt (BT45, BT46)
South Ockendon (RM15)
South Woodford (E18)
Upper Holloway (N7)
Watch: What you can and can't do during England's lockdown