Some 31 million Britons were hit by postal over Christmas as Royal Mail “let consumers down for a third year running,” Citizens Advice said.
Some 60% of UK adults experienced delayed post, with some still receiving festive cards well into the new year, a survey for the charity suggests.
Some 11% - an estimated 3.2 million - said they missed an important document, such as an insurance letter or legal document, and 8% - an estimated 2.1 million - said they missed a health appointment.
Citizens Advice said it was the third festive period in a row when it had called on the Royal Mail to improve its performance.
It previously found that 28% of people experienced letter delays between mid-December 2021 and mid-January 2022 and 31% were left waiting in January 2021, suggesting "deep-seated problems at play" despite postal strikes affecting performance this year.
The survey found that almost a quarter of people (23%) turned to more expensive ways of sending mail, such as Special Delivery Guaranteed, during Christmas 2022. However, 39% of those who did so still faced delays, despite paying extra.
Royal Mail has an obligation as Universal Service Provider to deliver mail on time, but it is exempt from its targets around the Christmas period.
Citizens Advice has now called on regulator Ofcom to investigate Royal Mail's performance in recent years and to “think again about not holding the firm to account over its performance during the festive period.”
It has also urged Royal Mail to scrap any further stamp price rises “at a time when performance is so poor.”
Citizens Advice chief executive Dame Clare Moriarty said: “For a third year running, Royal Mail has let consumers down. Letter delays have real and worrying consequences, especially when people miss medical appointments or get bills late.
“Royal Mail's virtual monopoly on letters means that Ofcom needs to take action to protect consumers from further harm. It must investigate Royal Mail's culture of poor performance and stop letting the company off the hook over the festive season.”
Yonder surveyed 4,148 UK adults between January 6 and 10.