People with glaucoma are going blind because NHS eyesight services have “inadequate capacity” to follow up such patients properly after diagnosis, an investigation has revealed.

An estimated 22 patients a month are suffering severe or permanent loss of sight because of long delays in getting follow-up appointments, the patient safety watchdog found.

The investigation by the watchdog, the Healthcare Safety Investigation Branch (HSIB), highlighted the problem after a 34-year-old woman lost her sight completely due to delays she experienced in seeing specialists over the course of 11 months. The woman, a mother of three, has not been named. She was awarded £3.2m by the NHS last year as compensation for the life-changing consequences of the delays in her treatment.

In a hard-hitting report the HSIB warns that patients who have glaucoma are regularly losing some or all their sight because hospital eye services are too oversubscribed to be able to give them follow-up appointments quickly enough.

Keith Conradi, the HSIB’s chief investigator, said: “Glaucoma is the world’s leading cause of irreversible blindness. We know that delay to appointments once patients are diagnosed exacerbates the risk of sight loss in patients across England.

“Our case highlighted the devastating impact. Our patient has suffered immeasurably, living with the effects each day, including not being able to see the faces of her young children or read books to them.”

The woman saw seven ophthalmologists after being referred to Southampton eye hospital in 2016 by an optician. She was given eyedrops and told she would have follow-up appointments. However, she did not have laser eye surgery for 11 months after her referral, by which time she already had severely impaired sight which could not be reversed.

A subsequent NHS inquiry found that 15 other patients at the same hospital had also either gone blind or suffered loss of sight because of delays, and that, in all, 4,000 glaucoma sufferers had not had appointments as quickly as needed.

Research in 2017 found that an estimated 22 patients every month suffer severe or total sight loss as a direct result of delays in their treatment.

Conradi criticised the NHS inaction over the problem, which has been evident for many years. “Despite some national recommendations being made 10 years ago this continues to happen and will only worsen as the population ages. A 44% increase in the number of people with glaucoma is predicted by the year 2035,” he said.

HSIB’s report said: “There is inadequate hospital eye services capacity to meet the demand for glaucoma services. A shortage of ophthalmologists is a particular problem.”

Helen Lee, policy and campaigns manager at the sight loss charity RNIB, said the HSIB findings had shown “a serious and dangerous lack of specialist staff and space in NHS ophthalmology services across the country”.

She added: “Thousands of patients are experiencing delays in time-critical eye care appointments, which is leading to irreversible sight loss for some. Suggestions on how to tackle the problem have been continually ignored. Without immediate action we’re very concerned that more people will experience avoidable sight loss.”

The HSIB has recommended a series of changes to reduce delays. Specialist eye doctors, especially locums and trainees, need to stop their “risk averse behaviour” in referring too many patients for a follow-up appointment, it said. And hospitals should give greater priority to those who have already been seen at least once, rather than new referrals, as they are at the greatest risk of their glaucoma taking away their sight, it added.

The NHS did not respond directly to the findings. A spokesman said only: “Fortunately the number of NHS-funded cataract operations is now at a record high, and by streamlining referrals and assessment as the NHS will be doing this year, it should be possible to further speed up access to expert eye care.”