Senior doctors have blown the whistle after almost 10,000 patients did not receive follow up appointments after treatment at Aintree Hospital - some as long as six years later.
The problem, which affects the gastroenterology department at Aintree Hospital, dates back to 2015 and has become a major concern for health bosses.
Clinicians have also contacted healthcare watchdog the Care Quality Commission (CQC), which is now looking into the situation.
Aintree Hospital's board merged with the NHS trust running the Royal Liverpool and Broadgreen hospitals in 2019 - creating a new organisation called Liverpool University Hospitals NHS Foundation Trust (LUHFT).
As part of the reorganisation, the gastroenterology departments at Aintree and the Royal are in the process of merging.
According to reports in the Health Service Journal (HSJ), consultants at the Royal came forward with serious concerns after discovering the extent of the problem at Aintree.
The problem affects patients who had received treatment at Aintree and were supposed to have follow-up appointments to ensure their recovery was on track and their conditions had not deteriorated.
However around 7,000 of the cases have "target dates" for an outpatient follow-up that have now passed without the patients being seen.
Around 20 of the patients were supposed to be seen in 2015/2016, around 400 in 2017, and around 900 in 2018, the HSJ reported.
Of the total number, around 2,500 cases either have no target date or have not yet had a follow-up appointment booked.
Around half of the cases are understood to involve patients with serious conditions including inflammatory bowel disease or liver conditions.
The problem was being discussed at the board of Aintree in 2018 before the merger with the Royal, at which point the waiting list backlog had reached around 6,500.
That had reduced by around 1,000 by the time the Covid pandemic rocked NHS services, and since then the backlog has rocketed to close to 10,000.
A spokesman for LUHT said a process was underway to review each affected case, and of 1,100 reviewed so far no patient harm had been identified.
The trust said a "shortfall in capacity" was identified due to "the cumulative effect of workforce gaps" after a review in 2018.
Around £720,000 was then spent to reduce the waiting list before the covid pandemic exacerbated things.
A statement from the Trust said: "A ‘deep dive’ review into Gastroenterology services was undertaken in February this year reflecting the high volume of work within this service, previous capacity challenges and the impact of the pandemic.
"The Trust is working through the findings of the review and has an action plan in place to ensure patients receive the care they need as soon as possible."
A CQC spokesman said: “We have been made aware of concerns regarding waiting lists at Aintree University Hospital’s gastroenterology services and we are following these up with the trust."
