Royal Mail is being investigated by Ofcom after the service failed to deliver less than three quarters of first-class post on time in the last year.
In its yearly financial results on Friday, Royal Mail’s parent company, International Distribution Services (IDS) said only 74.5% of first-class mail was delivered within one working day.
Regulator Ofcom’s rules state 93% of first-class mail must be delivered within the timeframe, excluding Christmas.
Ofcom said: “If it does not provide a satisfactory explanation and we determine Royal Mail has failed to comply with its obligations, we will consider whether to impose a financial penalty.”
Last year, Ofcom fined the company £5.6m for failing to meet its delivery targets in 2022-23.
Royal Mail’s released figures were delivered more than 24 hours late, after markets closed on Friday.
The chief executive of IDS said the financial results showed Royal Mail’s losses narrowed to £348m – from £419m for the year ending 31 March.
Martin Seidenberg said: “We have improved quality, won back customers lost during industrial action, controlled costs and delivered Christmas for our customers.”
The results come as IDS waits for a potential buy-out offer from Czech billionaire Daniel Kretinsky.
It reiterated it is “minded” to accept a proposed bid worth about £3.5bn put forward on 15 May.
Business Secretary Kemi Badenoch said earlier this month that Royal Mail’s universal service obligation will need to be protected in any sale of the firm.
IDS has previously said Mr Kretinsky agreed to offer a set of “contractual undertakings” to protect key public interest factors.
It also said the proposal recognised Royal Mail’s status as a major part of national infrastructure.
This would include commitments to Royal Mail’s plans to keep six-day-a-week first-class letter deliveries under the universal service, protect workers’ rights and keep the Royal Mail brand, as well as its UK headquarters and tax residence.
Mr Kretinsky has until 29 May to make a firm offer.