“We’re working hard to deliver as comprehensive a service as possible with the resources available to support these offices.
“And we’re ensuring we prioritise the delivery of COVID-19 vaccination letters and test kits.”
Delays can be caused by factors such as poor weather conditions, increased demand and resourcing issues.
The delivery service explained resourcing issues happen when colleagues self-isolate in line with Government guidance.
It said: “During the coronavirus pandemic, we’re doing all we can to keep our people and our customers safe while keeping the UK connected.