A cancer patient missed an appointment after receiving the letter three days after the consultation was due to take place.
Peter Barry, 75, of Castlefields, Runcorn, said residents in his neighbourhood have been receiving letters from once a week to 10 days.
This meant his letter notifying him of a consultation in relation to his “low-grade” cancer arrived three days after it was supposed to take place.
Mr Barry said three people he knows had told him they were experiencing similar delays.
His concern comes after residents in Weston Point, another Runcorn neighbourhood, recently told the ECHO their post had been disrupted.
Many expressed support for Royal Mail postal workers having to contend with the workforce pressures inflicted by the pandemic.
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Despite understanding the strains incurred by coronavirus, Mr Barry’s situation raises questions about keeping patients informed amid a stretched postal service.
Royal Mail has not responded to a request for comment but a general statement on its website says its staff are working hard in “unprecedented times” and have to deal with coronavirus-related absences and social distancing measures.
It has also limited some services such as its next day special delivery guarantee, and it launched a recruitment drive in October to fill hundreds of roles.
Wales Online has reported the firm has been struggling to handle “exceptionally high volumes” and had taken on 33,000 extra seasonal staff.
Mr Barry was concerned that people could be missing out on receiving their letters offering a Covid-19 vaccine as early as possible.
He said: “I got 10 letters in one and I’d had nothing for 10 days.
“In there was a letter for a telephone consultation for three days previous.
“I’m wondering if they’re sending people letters for them to get their Covid vaccine.”
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He added: “I did phone them up later and I wasn’t able to speak to the consultant.
“It was just a check-up, it could have been important.
“We’re only getting it (post) once a week.”
In its general online statement, Royal Mail said: “We continue to work hard to collect, process and deliver the UK’s mail in unprecedented times.
“To manage the effects of Coronavirus-related absences and necessary social distancing measures such as reducing the number of people together in shared spaces and vehicles, we have implemented a range of mitigations.
“These include changing the time guarantee on special delivery and opening hours of customer service points at local delivery offices.
“Our delivery operation is a key part of keeping the country moving.
“In fact, we have put even more resources in to keep delivering.
“This includes additional overtime provision and a very significant investment in protective equipment for our colleagues.
“We thank you for your support and patience.”