Royal Mail is to trial a Sunday parcel delivery service for major retailers this year.
The move is aimed at tapping into the seven-days-a-week delivery market as more consumers expect Sunday deliveries as part of their online shopping.
In the last year, Royal Mail said it has processed unprecedented parcel volumes, delivering 496million in the third quarter ending December 27.
Royal Mail said construction of its second, and largest, parcel hub is under way in Daventry in Northamptonshire, with the capacity to process more than one million parcels a day.
Chief commercial officer Nick Landon said: "The UK already trusts us to deliver their purchases six days a week both quickly and conveniently.
"Now, for the first time, our posties will be doing the same thing seven days a week.
"The last year has reset so many customer expectations and the desire for even more convenient and even more frequent parcel deliveries has certainly been one of them.
"We always listen to our customers, both senders and recipients, and the ask here was clear: we love what you do Monday to Saturday, so please do the same on a Sunday.
"So that's what we're doing, as quickly as possible, so we can offer it to more and more customers across the course of this year."
Royal Mail came under fire last year after it emerged the service could scrap Saturday post.
The privatised postal giant is required by law to deliver six days a week under what is called the universal service obligation.
But the firm said it had seen a slump in the number of letters it handles because of emails, more billing done online and changing habits.
It said the coronavirus lockdown saw Royal Mail deliver 1.1billion fewer letters.
Keith Williams, interim executive chairman at the Royal Mail Group, said in a letter to staff at the time: "These findings tell us the best way to ensure the 'universal service' continues to meet our customers' needs is to rebalance our service model more towards the growing parcels market, particularly urgent parcels, and urgent 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."
