Royal Mail could be allowed to end the delivery of second class letters on Saturdays, as part of reforms being considered by the regulator.
Ofcom will assess whether changes to the second class service are needed, while first class deliveries remain at six days a week.
The regulator has been reviewing the Universal Service Obligation under which Royal Mail is required by law to deliver letters six days a week and parcels five days a week.
The number of letters sent through the post has been falling steadily for years while parcels volumes have grown, leading to losses at Royal Mail.
Lindsey Fussell, Ofcom’s group director for networks and communications, told the BBC's Today programme the universal service “does need to change”.
“To be clear, no change is not really an option, otherwise it is going to be unsustainable and we’re all going to have to pay a lot more for it,” she said.
Royal Mail's owner, International Distribution Services, said that “change cannot come soon enough”.
IDS chief executive Martin Seidenberg said: “The universal service faces a very real and urgent financial sustainability challenge."
IDS is currently in the process of being bought by Czech billionaire Daniel Kretinsky for £3.57bn.