Royal Mail could reduce delivery of letters to five days a week by dropping Saturday, or even cut deliveries to as little as three days, according to proposed reforms.
The changes proposed by Ofcom said Royal Mail could save around £100m to £200m by switching from six to five days a week, or £400m to £650m by moving to three days.
Another proposal is making changes to existing first class, second class and business products so that most letters are delivered through a service taking up to three days or longer, with a next-day service still available for urgent post.
Dame Melanie Dawes, Ofcom's chief executive, said:
"Postal workers are part of the fabric of our society and are critical to communities up and down the country. But we're sending half as many letters as we did in 2011, and receiving many more parcels. The universal service hasn't changed since then, it's getting out of date and will become unsustainable if we don't take action.
"So we've set out options for reform so there can be a national discussion about the future of universal post. In the meantime, we're making sure prices will remain affordable by capping the price of second class stamps."
Under the universal service obligation (USO), Royal Mail is legally required to deliver letters six days a week and parcels from Monday to Friday, but the number of letters being sent has dropped significantly and Royal Mail says the current system is "not sustainable" and reforms are "urgently needed".