Royal Mail was separated from the Post Office and privatized 10 years ago, but its performance has deteriorated in recent years, leading to significant financial losses.
The amount of letters sent in the UK has plummeted, with the number now half as much as it was in 2011.
Royal Mail was fined £10.5m by regulator Ofcom last week for failing to meet delivery targets for first- and second-class mail.
Ofcom’s Dame Melanie Dawes told the BBC it was up to the new owners to deliver improvements and that the regulator would “absolutely” hold Royal Mail to account.
Jenny Hall, director of corporate affairs at Royal Mail, told the BBC that the company was investing to improve its performance, but that USO would not be reformed to reflect consumer trends. “It was very important,” he said.
He added that while Royal Mail always strives to keep postage costs as low as possible, prices “need to reflect the realities of service provision”.
The price of the second class stamp, which currently costs 85p, is regulated by Ofcom, which sets the amount by which Royal Mail can increase the price each year in line with inflation.
But there is no such restriction for first class stamps, and in October Royal Mail increased the price of first class mail by 30p, citing “very real and urgent” financial challenges. The price was set at 1.65 pounds.