Royal Mail delivered less than 80% of first-class post on time in recent months, increasing the pressure on managers at the company, which is already being investigated for poor service.

The company said 79.1% of first-class mail arrived within one working day in the three months to 30 June. It also delivered only 94.1% of second-class post on time.

The figures lag annual targets set by the communications watchdog, Ofcom, which said Royal Mail’s performance was “not good enough”. Under its rules, 93% of first-class post must be delivered within one working day and 98.5% of second-class post must be delivered in three days.

Royal Mail said 93.5% of first-class letters arrived within two days, a small improvement on the corresponding figure for last year, of 91.6%.

In May, Ofcom said it was investigating Royal Mail for missing its delivery time targets for the year to March 2024.

The watchdog fined the company £5.6m for failing to meet its first- and second-class delivery targets in the financial year ending March 2023. In January, Royal Mail hailed its best Christmas for deliveries in four years but still missed a target that would have handed postal workers a £500 bonus.

Royal Mail’s owner, International Distribution Services, is in the process of being bought out by the Czech billionaire Daniel Kretinsky’s EP Group in a £3.57bn takeover. But the deal is not yet completed and the Cabinet Office has launched a review process into the bid under the National Security and Investment act.

Royal Mail said it had taken steps to improve its delivery time, including employing more permanent staff, bringing in new rules to cut down on sick days and improving retention. It has also established a quality control centre to manage disruptions.

The 507-year-old company is lobbying Ofcom to be allowed to reduce deliveries of second-class letters to just two or three days a week, cutting nearly 1,000 jobs and saving £300m a year in the process.

A spokesperson for Ofcom said: “Royal Mail’s performance hasn’t been good enough. Last year, we fined the company and we’re currently investigating its latest failure to hit its annual delivery targets.

“We’ve been pressing the company on what it’s doing to turn things around. Royal Mail recently published an update on its improvement plans. It’s made some progress recently, but there’s still more to do. We’ll continue to hold Royal Mail to account, and take action on behalf of its customers.”

Royal Mail’s chief operating officer, Alistair Cochrane, said: “Delivering a high quality of service continues to be our top priority and I am pleased this ongoing focus has ensured improvements across the quarter.

“We know there is more to do and will continue on this path, enacting steady changes to our operational model to bring long-term improvements to our quality of service.”