An estimated 15.7 million people in the UK experienced postal delays last month, according to new research commissioned by the charity, Citizens Advice.
Many of those who experienced delays said they had suffered knock-on impacts, such as missing health appointments, fines or bills.
One woman said at least four of her hospital appointment letters were delayed during a “high risk” pregnancy.
Citizens Advice Chief Executive, Dame Clare Moriarty, described the level of delays as “appalling”.
The charity also called on regulator Ofcom to strengthen its current review of postal services.
Royal Mail said the year 2022/2023 was “one of the most challenging in our history” and said its services had been impacted by strikes and “high levels” of staff absence.
The survey of more than 4,000 adults surveyed between 25 May and 5 June found nearly one in three (31%) of those questioned – equivalent to be around 15.7million people if replicated across the UK – said they had experienced a letter delay, while 22% said they had experienced a parcel delay.
Of those who responded, 15% said they had experienced a serious negative consequence, including missing important documents, missing a health appointment, or losing money through fines.
The charity also said that its research showed how people of colour were nearly twice as likely (23%) to experience negative consequences as a result of letter delays compared to white respondents (13%).
Meanwhile, 21% of disabled people experienced negative consequences as a result of letter delays, compared to 13% of non-disabled people, according to the charity.
Winifred, a 24-year-old from Hemel Hempstead, told the charity that during her pregnancy – regarded as “high risk” by doctors – she waited for multiple hospital letters that failed to arrive on time.
“I was so stressed out,” she said.
Winifred, who now has an eight-week-old baby, added: “Another time, I knew I had an appointment that week, but hadn’t received the letter so I went directly to the hospital to ask when the appointment was.
“They told me it was the next day – if I hadn’t gone to the hospital to ask, I would have missed it.”
The Citizens Advice report comes after MPs recently highlighted evidence that Royal Mail had prioritised parcels over letters and called on Ofcom to investigate this issue across a number of years.
The charity said its research showed it was no longer acceptable for Ofcom to have a business-as-usual approach to its investigation and called on the regulator to launch a multi-year review into mail delays and deprioritisation.
Dame Clare Moriarty said: “Royal Mail’s delays are still at appalling levels and it’s consumers who are being saddled with the consequences.
“Delayed post’s been an issue for years and the problem is only getting worse. Ofcom must now do a full root-and-branch investigation into mail delays.”
A spokesperson for Royal Mail said: “We’re sorry to any customers who may have been impacted by our performance during a year that has been one of the most challenging in our history, with quality of service materially impacted by the long-running industrial dispute with the CWU and compounded in some areas by high levels of staff absences.
“Improving quality of service is a top priority and an improvement plan is already underway.”
An Ofcom spokesperson said: “We assess Royal Mail’s performance against annual delivery targets and we are investigating its failure to meet delivery targets for 2022/23.
“We take quality of service seriously. If we determine that Royal Mail has failed to comply with its obligations, we may consider whether to impose a financial penalty.”