A serious incident has been declared at a struggling foundation trust for the second time in 12 months after more than 20,000 outpatients were overdue follow-up appointments.

Northern Lincolnshire and Goole FT was warned by the Care Quality Commission in its April inspection report about “unacceptable” delays after 30,000 patients had overdue consultations last year.

Similar problems have now emerged weeks before the trust faces a CQC inspection next month. The trust runs hospitals in Grimsby, Scunthorpe and Goole.

More than 22,600 people last week were overdue outpatient follow-ups, down from 26,600 at the end of August. Hundreds of appointments are more than eight months beyond their due date.

Ophthalmology patients are the worst affected, with a backlog of 3,300 last week, down from 5,700 a month earlier.

Working through the backlog is having a knock-on impact on the trust’s 18 week inpatient treatment target, prompting it to source capacity from alternative providers, under a recovery plan drawn up for NHS England.

The trust told the CQC it had cleared the original backlog but waits have grown again this year due to continuing staff shortages.

A report to North East Lincolnshire Clinical Commissioning Group’s partnership board last month said commissioners had made “numerous requests” for updates about ophthalmology performance in the light of previous concerns but only received data in July.

It said: “The trust admitted while it was clearing the original backlog further patients who then became due for an appointment were not added to the original cohort due to continued capacity constraints.”

The trust declared a serious incident and the issue had “culminated” with NHS England’s regional quality safety group meeting with the trust, the report said.

NHS Improvement’s intensive support team is working with the trust and it has re-hired improvement director Eric Morton to provide independent scrutiny ahead of the CQC inspection – two years after he left the post created after the trust was put into special measures over concerns about mortality rates.

Karen Fanthorpe, the trust’s interim chief operating officer, said: “We have agreed a recovery plan with our commissioners to increase service capacity, including hiring more locum doctors, providing additional clinics at weekends and evenings, creating additional clinic rooms, and introducing technicians to help with diagnostic tests.”

In April, the trust was rated requires improvement by the CQC but outpatient services were rated inadequate. Inspectors said there had been seven serious incidents in outpatients leading to patient harm between September 2014 and October 2015, including delayed treatment due to cancelled appointments and failure to follow up in a timely manner.