Royal Mail has been given two weeks to respond to allegations – reported by the BBC - that it is prioritising parcel deliveries over letters, resulting in delays across the postal network.
In a letter to the company, the Business and Trade Committee of MPs asked for commitments to improve what it described as "chaos".
The letter comes after hundreds of people contacted the BBC to claim parcels were being prioritised while letters were being stored up so that they could be delivered in batches.
Royal Mail said it would respond to MPs in detail, but that it understood the frustration of post not arriving as expected. It said it was working to resolve the disruption.
Speaking to BBC Breakfast, Liam Byrne MP, chair of the Business and Trade Committee, said he had seen the comments made by viewers and wanted "reassurance" from the company that things would improve.
He said: "Is there any truth to the stories that they are prioritising parcels instead of letters?"
"Is there any truth that they're batching up letters in big piles before they embark on deliveries to houses?
"We're going to need to see a plan on the table now to reassure us that this service is going to be stood back up to what the nation expects."
In the letter to Royal Mail, the committee raised "significant concerns" about reports of "failures in service" that go beyond normal seasonal pressures.
The letter demanded "categorical assurance" that parcels are not being prioritised over letters, and gives the company two weeks to provide commitments to address the "chaos".
Royal Mail told the BBC it only prioritised parcels when it was necessary to do so in order to clear bulky items from sorting offices for health and safety reasons. It said it was normal for people to receive a number of items at the same time, creating a perception of so-called "batching".
The regulator, Ofcom, has fined Royal Mail £37m in recent years for poor performance delivering letters.
Following the publication of the letter from MPs, Ofcom told the BBC it expected to see meaningful change from Royal Mail soon and that, if that does not happen, "fines are likely to continue".
The letter came after BBC News spoke to more than a dozen Royal Mail staff who say rounds are being missed on a daily basis and some first-class letters are sitting in delivery offices for weeks.
The company has admitted there are service delays in over 100 postcodes, but it said this was due to storms and higher rates of illness among staff.
Last year, Royal Mail was fined £21m by Ofcom for failing to meet its delivery targets. It was the third such fine in as many years.
At the time, Ofcom warned that these fines were "likely to continue" unless the company urgently delivers "a credible improvement plan".
Royal Mail, which is privately owned by a Czech billionaire following a recent takeover, is legally required to deliver letters every day in most areas.