Royal Mail has been referred to industry regulator Ofcom after MPs accused the firm of failing to deliver letters six days a week.
A group of MPs from the Business, Energy and Industrial Strategy Committee said the firm has been prioritising parcel deliveries over letters.
Royal Mail should deliver letters across the country six days a week as part of its "universal service obligation" - but the committee said it has been "systematically failing to deliver" on this.
The committee also said Royal Mail chief executive Simon Thompson was "not wholly accurate" in answers he gave to MPs on the use of technology to track and discipline workers.
MPs said they "did not believe that such widespread errors could happen without direct or indirect approval of management".
Committee chairman Darren Jones said: "I find it hard to believe that such widespread breaches of company policy and legal obligations are down to a national network of rogue workers conspiring against management at Royal Mail.
"We were inundated with evidence from postal workers challenging the accuracy of answers given by Royal Mail CEO Simon Thompson."
The committee said Royal Mail was going through "troubled times" and noted the firm said last year it was losing £1 million per day.
The group added: "It is in the midst of a bitter industrial dispute with its workforce and we believe that Royal Mail is failing to meet some of its statutory requirements under the USO.
"When we explored these issues with Royal Mail's senior management, our concerns were not allayed, in fact they grew.
"Some of the statements made by the chief executive officer during oral evidence provoked a huge response from postal workers, who contacted the Committee directly, claiming that the Committee had been misled."
In response, Ofcom said the recent performance from Royal Mail was "clearly well short of where it should be" and confirmed it would consider the MPs' report.
A Royal Mail spokesperson said: "We have informed the committee that we will be reviewing the consistent application of our policies regarding the delivery of letters and parcels across the business.
"We will share our findings with the committee and Ofcom. We have asked the committee to share the material they have received, and reiterate again our request for them to do that at the earliest opportunity so it can help inform that review.
"Royal Mail answered in detail the questions asked by the committee - in person and in correspondence - about the company's performance, finances and service delivery.
"We reject the suggestion that Royal Mail may have misled the BEIS Select Committee in that process."
Communication Workers Union general secretary Dave Ward said: "The report vindicates why the workforce have lost total confidence in CEO Simon Thompson and his senior leadership appointees.
"Their mantra of 'our business to run' has seen a vicious and unprecedented level of attacks on postal workers and the service they provide.
"The deliberate running down of the universal service obligation has actually worsened, not improved, since the Select Committee."
