February figures from NHS England showed that 31.3% of people attending A&E departments run by Barking, Havering and Redbridge University Hospitals Trust had to wait more than 12 hours.
A total of 10,690 people attended the trust’s A&E departments in February, and it took more than 12 hours for 3,350 of these patients to be admitted, transferred or discharged.
No other London NHS trust saw more than one in five people wait this long.
Nationwide, around 125,000 people are thought to have waited more than 12 hours at A&E departments in February.
Within the NHS, the operational standard is that at least 95% of patients attending A&E should be discharged within four hours.
Miriam Deakin, director of policy and strategy at NHS Providers, the membership organisation for NHS trusts in England, said the new data on A&E waits was “a real concern” which reflects “the incredible demand-driven pressures on trusts”.
She added: “The NHS is under severe and unsustainable pressure. Persistent challenges across urgent care are hindering progress on ambulance response times and A&E waits.
“Trust leaders are desperately awaiting a long-term fully funded workforce plan, which should start to address staff shortages, exhaustion and burnout.”
Dr Tim Cooksley, president of the Society for Acute Medicine, said emergency care faces “extremely troubled times ahead” and the findings are “of grave concern.”
He added: “We are heading for extremely troubled times ahead in urgent and emergency care and this data is a warning of what is to come.
“Overcrowding in emergency departments (EDs) and acute medical units (AMUs) means many patients are still not receiving timely and high-quality patient care.
“Caring for patients in such inappropriate environments not only provides a poor and undignified patient experience, it poses significant risks including increased risk of severe, and sometimes fatal, harm.”
Dr Layla McCay, director of policy at the NHS Confederation, said pressure on the service continues to be “relentless”.
She added: “The figures for 12-hour A&E waits from arrival show that too many patients are having to wait longer than NHS leaders would want or expect.
“This is an ongoing priority for NHS leaders and their teams and has been addressed following the steps taken in the recent urgent and emergency care recovery plan.”
Some reporting by PA