Royal Mail says it is facing disruption at some of its delivery offices, with customers in south east London among those affected.
The delays, impacting areas including East Dulwich (SE22) and Herne Hill (SE24), are the result of ongoing issues such as high levels of staff sickness and resourcing challenges.
Royal Mail has said these problems are affecting a "small number of local offices" and that it is "sorry for any inconvenience" caused to customers.
Royal Mail 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 high levels of sick absence, resourcing, or other local factors.
"In those cases, we will rotate deliveries to minimise the delay to individual customers.
"We also provide targeted support to those offices to address their challenges and restore our service to the high standard our customers would normally receive.
"We're sorry for any inconvenience and thank you for your understanding."
Royal Mail has said it is actively managing the situation by rotating deliveries in the affected areas to reduce delays for individual customers.
Targeted support is also being provided to those offices to address their challenges and restore service to the high standard customers would normally receive.
Royal Mail regularly publishes updates on its website listing the offices currently facing the greatest levels of disruption.
Customers who have received a 'Something for You' card are encouraged to check the Royal Mail website for the latest information on Customer Service Point locations and opening hours.