The postal regulator has defined five key areas of service that Royal Mail needs to provide at an affordable flat rate as a universal postal service.
These are first and second class mail, a standard parcel service, special and recorded delivery, international outbound mail and a mail redirection service.
Postcomm's areas of service do not include a universal obligation to provide direct mail or newspaper delivery.
Postcomm chairman Nigel Stapleton said: "This is another important step towards opening up the postal market to competition and ensuring customers get the products they want.
"Until now it was assumed that the universal service applied to everything that Royal Mail did. Postcomm's decision means that the legal definition of the universal service is now clearly translated into the postal service that customers want."
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