Hundreds of people have complained as letter deliveries in one town are taking up to four weeks to arrive.
People in Bridgwater, Somerset, have been missing key appointments or waiting weeks for bank cards to arrive, with the town's MP calling for Ofcom to urgently review whether Royal Mail's "chronically understaffed" team is a breach of its licence.
Resident Jayne Mogg organised a public meeting about the issue on Saturday and said people are very frustrated.
A Royal Mail spokesperson said: "Bridgwater Delivery Office is currently experiencing resourcing pressures and we are taking action to address these."
Mogg nearly missed a medical appointment with a consultant because her letter was not delivered. She claims she only knew about it because the hospital texted her to change the time.
She said she first noticed there was an issue in January, but believes things got worse over the summer.
"Sometimes I get a massive wodge of post and then nothing at all," Mogg said.
She set up a social media page when she realised several other people in her village were struggling to get post too.
Generally she said parcels are coming through, but letters are still being delivered in bundles that come every three to four weeks.
Mogg said she is passionate about sorting this out, particularly for the vulnerable and immobile people in her village, who she said are paying for a service they cannot rely on.
Mogg was keen to emphasise that the meeting was also organised to show support to Royal Mail staff, which she described as a "British institution".
She said they want more specific data from Royal Mail on current staffing, a public apology and are considering applying for compensation.
Sir Ashley Fox, MP for Bridgwater, said he was told in a meeting with Royal Mail that there were 17 vacancies at the town's depot.
"You don't get to be 17 staff short by accident," he said.
Speaking on BBC Radio Somerset, he said he has called on Ofcom to urgently review the Royal Mail's contract as he believes it might be breaching the terms of its licence.
He said he has had more than 200 constituents contact him, some complaining that they are routinely waiting up to four weeks for their post.
Some, he said, have resorted to queuing three or four hours at the sorting office to collect their mail, only to find that the item they want is still in an unsorted sack.
An Ofcom spokesperson said the authority shares "the deep frustrations of MPs and their constituents with Royal Mail's poor delivery performance, which hasn't been good enough".
"We're currently investigating the company's failure to meet its latest targets, and have fined it more than £37m in recent years.
"We also demanded that Royal Mail publish a credible improvement plan, backed by investment from its owners. In response, the company committed to return performance back to the regulated targets by April 2027, and to invest £500 million over the next five years," they said.
Royal Mail chiefs said they are holding local recruitment events and hiring agency support to "strengthen the team while permanent recruitment continues".
"Where individual routes are affected, our teams work to deliver any outstanding mail as quickly as possible.
"We know there is more to do and are continuing to make changes to provide customers with a more reliable and consistent service," they added.