One in five letters and parcels sent first class do not arrive the next working day, Royal Mail has admitted.

Just weeks after it increased its postage prices, when the cost of a first-class stamp went up 10p to 95p, the company said its services had been hit by the continued impact of Covid and higher staff absences.

Royal Mail said just under 82% of first-class mail was delivered the next working day in the 12 months to the start of April. This was an improvement on the level of almost 75% a year earlier, but still below its key target of 93%.

Its second-class deliveries fared better, according to its annual service statistics, and 95% of these items arrived as expected within three working days. But this was still below the company’s target of a minimum of 98.5% second-class items delivered within this timeframe.

Royal Mail said the quality of its service during the year to April was “materially impacted by the exceptional ongoing effects of the pandemic resulting in increased Covid-related absence and self-isolation and social distancing in line with government guidance”.

The London-listed company is required to deliver to 31m homes and business across the UK six days a week for the same price, regardless of the distance travelled, under its universal service obligation.

It said far more parcels and far fewer letters were sent during the pandemic, which made it “very challenging” to meet its usual service standards.

Absences among Royal Mail staff hit a peak in January, when more than 15,000 employees were off sick or were self-isolating, as the Omicron variant spread across the country.

The company said it recruited temporary additional staff and set up a special delivery task forces to provide support to the offices that were most affected by staffing shortages.

There have been improvements to its service in recent weeks, the company said, with more than 86% of first-class mail arriving the next working day, and more than 97% of second-class items within three working days.

Royal Mail has previously blamed a long-term decline in letter usage and rising costs for its most recent inflation-busting postage price increases, which came into effect in early April.

The company has also been facing growing complaints from customers, which soared to more than 1m in the year to March 2021, the highest level for a decade.