Royal Mail wants to drop Saturday deliveries but the prime minister says they are “important”.
The government has said it would not support Royal Mail scrapping post deliveries on Saturdays.
The company has said it would like deliveries from Monday to Friday only.
But Downing Street said the government “would not countenance” such a move, as a review by regulator Ofcom is looking into the future of the postal service.
“The PM’s strong view is that Saturday deliveries provide flexibility and convenience,” said Rishi Sunak’s spokesperson on Monday.
“They are important for businesses and particularly publishers. The Prime Minister would not countenance seeing Saturday deliveries scrapped.”
Ofcom, which regulates the postal industry, is set to publish a document outlining options on how Royal Mail can “evolve to more closely meet consumer needs”.
The regulator has not commented on what the options will be.
Royal Mail posted a £319m loss for the first half of the current financial year and the business, which is owned by shareholders, has been calling for urgent reform since 2020.
It has said previously that it would like to go from delivering six days a week to five, from Monday to Friday only.
Ofcom has said that while it will publish options for how the postal service might need to change, it would “ultimately be for the UK government and Parliament to determine whether any changes are needed to the minimum requirements of the universal service”.
The legal basis for the universal service is the Postal Services Act 2011.