An NHS trust has more than 1,000 patients who have waited over a year for treatment.
Barking, Havering and Redbridge University Hospitals Trust has a waiting list bigger than for the rest of NHS hospitals combined.
In a document seen by the Health Service Journal (HSJ), 1,015 patients had been waiting more than a year for planned treatment as of the start of February.
The trust has not reported waiting times data to NHS England since February 2014.
NHS England data shows there were 748 patients waiting more than a year across England for treatment as of December, although not all trusts have submitted data.
Of these, North Bristol and University Hospitals of Leicester accounted for almost half of cases.
In a statement, Barking's chief executive Matthew Hopkins said: "It is crucial that patients are seen as quickly as possible and we are sorry that a lot of patients are waiting too long for treatment.
"We are taking urgent action, working with our local partners and stakeholders, to ensure high quality, accessible care for our patients.
"We have significantly reduced the backlog of patients who have been waiting by putting on extra clinics to see and treat the people who had been waiting too long, but we know there is much more to do."
Mr Hopkins said that in early 2014 a new reporting system "identified several operational issues with our referral to treatment reporting, dating back several years".
He added: "We carried out a thorough validation exercise to make sure we were confident with our figures, and suspended reporting of our performance while that took place.
"A longstanding mismatch between capacity and demand, and the issues with reporting our performance, meant that a substantial backlog had built up.
"Those patients who have been waiting a significant amount of time have been contacted and we are booking in their appointments for the coming weeks."