Rishi Sunak opposes any end to Saturday deliveries by Royal Mail as part of an Ofcom review into the postal service, No10 said on Monday.
The watchdog will publish a document next week outlining options on how the loss-making Royal Mail can "evolve to more closely meet consumer needs".
"The Prime Minister's strong view is that Saturday deliveries provide flexibility and convenience," Mr Sunak's official spokesman told reporters. "They are important for businesses... the Prime Minister would not countenance seeing Saturday deliveries scrapped."
Royal Mail, which was privatised under the Conservative-led coalition government in 2012, posted a £319m loss for the first six months of this financial year.
The delivery company has been calling for reform since 2020 and has previously said a six-day postal service has become financially unsustainable.
Other ideas in the mix are a shift to deliveries on alternate days of the week, in line with with European markets such as Germany and Italy.
The Royal Mail’s universal service obligation (USO) requires it by law to deliver letters six days a week to all 32 million addresses in the UK for the same price no matter where the letters are going.
Ofcom announced in September it was reviewing the UK’s universal postal service regime.
A spokesperson for Ofcom told the BBC that it will publish evidence on "how the universal postal service might need to evolve to more closely meet consumer needs".
They added: "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 Government previously rejected a request from the Royal Mail to halt Saturday deliveries after a wave of strikes and a slump in demand for parcel deliveries.
Estimates suggested this would have saved the company hundreds of millions of pounds a year.
Ofcom said in November that the Royal Mail had "breached its obligations by failing to meet its targets by a significant and unexplained margin".
The watchdog added: "This caused considerable harm to customers, and Royal Mail took insufficient steps to try and prevent this failure."
In a letter to MPs this week, Martin Seidenberg, chief executive of Royal Mail's parent company, International Distributions Services (IDS), wrote: "Delivering the current Universal Service requirements - in a financially sustainable way - is increasingly difficult, if not impossible, to achieve as the mix and number of parcels and letters changes.
"The bar set by the regulations is unrealistic given the market realities."