Royal Mail has been fined £1.5m ($1.9m) for failing to deliver first class post on time.
The communications regulator also penalised the company £100,000 for overcharging customers after it raised the price of second class stamps too early.
Royal Mail missed its target to deliver at least 93% of first class post across the UK within one working day of it being collected.
Ofcom said in financial year 2018-2019, only 91.5% of first class post reached its destination on time.
The postal service also increased its price for second class post by 1p to 61p a week ahead of the official cap being lifted.
This led to overcharging of £60,000, the watchdog found.
Royal Mail admitted the mistake and donated the additional money to charity Action for Children.
The watchdog said it handed the hefty fine for the delayed post as the group did not provide a “satisfactory explanation and it did not take sufficient steps to get back on track during the year”.
"Royal Mail let its customers down, and these fines should serve as a reminder that we'll take action when companies fall short," said Gaucho Rasmussen, Ofcom's director of investigations and enforcement.
Royal Mail said it was “disappointed” with its first class service in 2018-19 and accepted the decision by Ofcom.
In a statement, it added: “We worked hard to restore our service quality in 2019-20 and, were it not for the pandemic and its impact on our business in the latter half of March, we were on course to deliver the requisite first-class regulated quality of service target of 93%.
“Despite our best endeavours, some areas of the UK experienced a reduction in service levels during March.”
Royal Mail announced last month that it would be cutting 2,000 management jobs to deal with the impact of the coronavirus crisis. The cuts will save the company around £130m.