“It would ultimately be for Parliament to decide whether this change was needed. However, Royal Mail must still modernise and become more efficient to keep pace with customers’ needs.”
A spokesman stressed that the report was research, and not a recommendation to Parliament.
The watchdog is separately reviewing the future regulatory framework for post in a review set to conclude in 2022.
Royal Mail interim executive chairman Keith Williams said in the last six months alone, letter volumes have fallen by around a third.
That has had a significant impact on the finances of the Universal Service which lost £180 million in the first half of the year.
He said: “To stay relevant and sustainable, the Universal Service must adapt to life in the 21st Century.”
He said Royal Mail was working to improve its efficiency levels.