The Business Secretary said she does not have plans to approve the Royal Mail switching to a five-days-a-week operation.
Kemi Badenoch suggested to MPs that allowing the Royal Mail to shed its Saturday delivery and pick-up duties would mean it “stops being the service which you want”.
The Royal Mail’s universal service obligation (USO) forces it to deliver letters six days a week to all 32 million addresses in the UK for the same price no matter where the letters are going.
Ofcom this month announced it is reviewing the UK’s universal postal service regime.
The watchdog said it would be gathering evidence on how the minimum service level “might need to evolve to more closely meet consumer needs”.
The Royal Mail, which was privatised under the Conservative-led coalition government in 2012, has lobbied for its obligations to be reduced, arguing the six-day service is unviable.
The firm blamed the lack of reform for having to increase the price of first class stamps, having announced it will put them up by 15p in October to £1.25.
Ofcom’s decision to review the USO comes only three months after the Department for Business and Trade said it would not support a change to a mid-week only service.
Junior minister Kevin Hollinrake wrote to the Business and Trade Committee in June saying: “We currently have no plans to change the minimum requirements of the universal postal service as set out in the Postal Services Act 2011, including six-day letter deliveries.”
Ms Badenoch repeated the position when she appeared before the committee on Tuesday.
Asked if the Government would accept an Ofcom recommendation for a switch to a five-day operation, the Cabinet minister said: “From my perspective, advice is advice.
“We have been lobbied on this before.
“We will look at the Ofcom advice but I certainly don’t have any plans to change the USO.”
Ms Badenoch added: “I don’t believe that the USO is the place to start in terms of looking at the Royal Mail’s business.
“It is, like the Post Office, incredibly difficult, and it is also difficult times for them to be operating.
“But I think if you start to move things like the USO, then it stops being the service which you want to be delivered.”
She said Mr Hollinrake was currently in discussions with the business to work out “how best to get themselves on an even keel”.
David Bickerton, a director general at the Department for Business and Trade, told MPs: “Ofcom has announced it is looking at the future changes to the USO and will produce options for views at the end of the year.
“So I think what we will do is we will get that advice and we will consider it.”
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