The company says there remains an "urgent" need for its Universal Service obligations to be relaxed, days after it was fined for poor performance.
Royal Mail's parent firm says the tough economy has knocked its recovery from the bitter pay dispute with frontline staff, reporting half-year losses of £319m.
IDS said the adjusted operating loss figure was 45% up on the same period a year ago and mainly driven by lower parcel volumes and the cost of the pay settlement with the Communication Workers Union (CWU).
The figures covering the six months to 24 September were revealed just days after the company was fined £5.6m for missing delivery targets, covering both first and second-class mail, over the 2022-23 financial year.
Industry regulator Ofcom, which imposed the penalty, described it as a "wake-up call" for Royal Mail which was crippled by industrial action during the period, especially in the run-up to last Christmas.
The dispute, which ran for almost a year, helped push Royal Mail into the red to the extent it was losing £1m a day.
The company racked up losses of £1bn in the financial year to 26 March amid a customer flight from the disruption.
IDS said on Thursday the Royal Mail was starting to win back business and that price rises had offset weaker volumes.
But IDS boss Martin Seidenberg reiterated the company's position that there was an "urgent" need for Royal Mail's Universal Service obligations to be relaxed.
It is currently required to deliver letters six days a week.
Mr Seidenberg said: "Looking ahead, we are transforming our business every day, but we can't do it all on our own.
"We also need the regulator and the government to do their bit.
"It's simply not sustainable to maintain a network built for 20 billion letters when we're now only delivering seven billion.
"The UK is not immune to the trends that we see across the world. Many other comparable countries have already reformed their Universal Service, and the UK is getting left behind.
"We welcome the fact that Ofcom will be reviewing options for the Universal Service, but the need for reform is urgent."
He added that Royal Mail was "pulling out all the stops" to deliver Christmas for its customers, saying his priority was improving quality.
IDS reported a group adjusted operating loss of £169m compared with £57m a year earlier.
It said it remained on track to break even for the year as a whole, due to revenue growth at its international parcels network GLS.
A rise of almost 6% at GLS was offset by weakness at Royal Mail, which saw parcel revenues fall 6.2%.
