Royal Mail has acknowledged that its performance "needs to improve" after admitting that a fifth of items sent first-class are no longer arriving the next day.
The delivery giant aims to deliver the majority of first class letters the following day including Saturdays - and traditionally has met this target.
However, it's now said one in five 95p-a-stamp letters are not landing through letterboxes within 24-hours due to Covid.
Royal Mail said its service standards had been "materially impacted" by the pandemic and workers self-isolating over the past year.
Reporting its annual quality of service statistics, the company said that 81.8 per cent of first-class mail arrived the next day in the year to April against its target of 93 per cent.
The figure is an improvement on the 74.7 per cent of letters delivered on time at the height of the pandemic, but is still likely to anger customers for whom the cost of a first-class stamp has risen to 95p.
Grant McPherson, the chief operating officer at Royal Mail, said: "We know that we need to improve our performance and some of the steps we have taken to address the issues are bearing fruit.
“Our transformation programme is introducing new ways of working, and a new delivery model will optimise our network for parcels."
Royal Mail hiked the prices of first-and second-class stamps earlier this year.
The cost of a first-class stamp went up by 10p to 95p, while second-class stamp prices rose 2p, to 68p.
Royal Mail said prices went up because Brits are sending fewer letters, and because inflation is so high, increasing its own costs.
The postal service said if it didn't put up prices then it could not continue its 'universal service'.
This is the promise that it costs consumers the same price to send a letter to any address in the UK, six days a week.
Royal Mail came under criticism last year after suggesting it could scrap Saturday post – a move that would break its universal service obligation.
It said the coronavirus lockdown saw Royal Mail deliver 1.1billion fewer letters.
But campaigners warned axing first class post could hit the most vulnerable people most.
Jan Shortt, of the National Pensioners Convention, said: "Older people, particularly those living alone, as well as other vulnerable groups unable to access online services, will be the most affected by this decision.
"Like those delivering milk and other goods to homes across the country, postal workers are also the eyes and ears of communities, making sure that those known to be alone and vulnerable are staying safe and well.
"Older people, more than any other group in society still rely on the post as their preferred method of their communication.
"Royal Mail may see this as a necessary cost cutting exercise and that one day out of six without post is insignificant, but we would ask them to consider those living alone and that receiving a nice letter, card or parcel on a Saturday morning actually gives them something to look forward to and sees them through until the beginning of the week."
Separately the postal service has agreed to push back on its plans to set a hard deadline for households to use up their old stamps.
Its original plan was to give Brits until January 31 next year to use up any existing Royal Mail stamps - including seasonal ones such as Christmas stamps, before a new barcode-only stamp system came into force.
