Royal Mail has failed to meet delivery targets for years and has previously been issued a £1.5m fine.
Royal Mail is again being investigated by the communications regulator over its missed delivery targets.
Ofcom announced the inquiry after Royal Mail said on Monday it did not meet its 2022 to 2023 quality of service targets.
The targets require Royal Mail to deliver 93% of first-class mail within one working day of collection; deliver 98.5% of second-class mail in three working days of collection; and complete 99.9% of delivery routes on each day that a delivery is required.
But the former state-owned monopoly said only 73.7% of first-class mail arrived within one working day; 90.7% of second-class post arrived within three working days; and just 89.35% of delivery routes were completed for each day a delivery was required.
As part of the investigation, Ofcom will examine if there were any reasonable grounds or exceptional events, beyond the company's control, for it to have failed to meet its universal service obligation.
A fine may be imposed if Royal Mail does not provide a satisfactory explanation, the regulator said.
Industrial action will likely be raised by the company as an explanation.
Over the past year, Royal Mail has been hit by strikes - before a settlement was reached last month - as staff members of the Communication Workers Union (CWU) staged walkouts, seeking improved pay and to retain working conditions.
In a contentious hearing, with MPs of the Business, Energy and Industrial Strategy (BEIS) committee, the Royal Mail boss admitted prioritising parcels over letters, during and following days of industrial action, though he denied that it was company policy to do so.
Simon Thompson had been recalled for questioning by the committee and was accused of giving "inconsistencies" in his evidence.
A Royal Mail spokesperson said on Monday: "We are disappointed with our quality of service performance and restoring our service to the high standards our customers expect is our top priority.
"We will participate fully with any Ofcom's investigation."
It's not the first Royal Mail investigation by Ofcom. In 2019 a £1.5m fine was issued for first class post delivery failures.
In December last year, the watchdog said it warned the delivery company it could not continue to use the COVID
-19 pandemic as an excuse for poor delivery after it did not meet the 2021 to 2022 delivery targets.
Ofcom did not find Royal Mail was in breach of any of its regulatory obligations for that year.
Royal Mail also did not meet 2020 to 2021 targets.
On Friday the Royal Mail chief executive formally announced his departure plan, as first reported by Sky News.
The company said it was in "advanced stages" of appointing a new chief executive and Mr Thompson will remain with the business until 31 October as part of the transition.