A woman who received an appointment for a major operation two weeks after it had taken place has described the situation with Royal Mail delays as "laughable".
Samantha Fisher is among residents on Wearside who have complained that vital health and legal letters are arriving up to a month after being sent, which she says is putting people's health "at risk".
Postal workers claim they are being told to prioritise parcels over the delivery of letters.
Royal Mail said it would "further investigate" the concerns raised and that undelivered parcels can cause delivery offices to "become very congested", creating safety risks.
The firm was fined £21m in October by regulator Ofcom after failing to meet its first and second-class delivery targets in the year 2024/25.
'Financial burden'
Mrs Fisher, who lives in Grindon, Sunderland, said: "They're messing with people's lives here, people's health."
She had been on a surgery waiting list at Sunderland Royal Hospital and only discovered an appointment had been set for 8 July after calling the hospital months after hearing nothing.
She was told a letter had been sent in June and a second was issued ten days later - but both arrived on 22 July while she was recovering in bed from treatment.
"I'd been on the waiting list so long, if I'd missed that appointment that would have been a massive financial burden to the NHS."
Then, after the delays, she said she only received a letter explaining her results and a treatment plan, dated 21 July, in the middle of August.
Customers have shown the BBC examples of "bundles" of a few weeks' worth of letters arriving in one go
Her husband Nigel said there had been problems with unreliable deliveries since April which had left him out of pocket.
This included a legal document which was only received three days after it needed to be responded to.
"Every time you've got to send an letter explaining the reason I'm late," he said, adding that he was spending about £9 each time to send a covering letter.
"If you want to send a letter, put in a box and send it by a parcel," he advised.
"They'll probably get it the next day".
In Washington, some people have seen letters significantly delayed or not received months after being sent.
Julie Harrison and her husband, who live in the Ayton area, said there had been a "massive shift" in the service provided since the summer.
"We are lucky if we get one or two deliveries a week now," she said.
Her mother Joan has always written to family members but replies stuck in the postal system are having a "massive impact" on her.
"It's an absolute shambles," Mrs Harrison said. "They haven't got enough delivery staff.
"I think there's a priority over parcels and letters will be pilling up in the sorting office."
The delays have lead to a "snowball" in complaints, according to the area's Labour Co-op councillor Brandon Feeley who revealed some constituents had missed deadlines for fixed penalty notices.
He said he only receives post once a week "if I am lucky" and urged the company to resume regular deliveries of letters.
One worker from a Wearside delivery office said it was "no secret" that Royal Mail was "simply putting parcels first and if there's time for letters, they will go".
"None of the post people want this... we have provided an amazing service for years, now our hands are tied."
Another worker said they were no longer able to defend the ongoing problems to customers who were frustrated and asking for explanations.
South Tyneside and Sunderland NHS Foundation Trust, which runs Sunderland Royal, said it was aware of the issues affecting some patients.
"Where necessary, we have spoken with these patients to rearrange appointments and make sure their care continues with minimal disruption," it said.
"The vast majority of our patients already receive their appointment letters via text message."
The trust said patients can continue to contact their appointment line if they "have any queries".
Regulator Ofcom said it had already fined Royal Mail £37m for its "poor letter delivery performance" and would continue to "hold the company to account".
In response to the Wearside concerns, a spokesperson for Royal Mail said the company wanted to "reassure customers that the overwhelming majority of mail is being delivered on time".
"Where a route is affected we prioritise any delayed items the following day.
"Our job is to deliver every item within the timescales customers have paid for, and our aim is always to deliver letters and parcels on time."
On the claims about prioritising parcel deliveries, the company said: "Clearing parcels first is sometimes necessary to keep offices safe and to ensure all mail, including letters, continues to move through the network.
"Ofcom has looked into this claim previously and found no evidence that Royal Mail instructs the prioritisation of parcels over letters outside of these recognised contingency plans."