The Nuffield Trust health think tank said delays to care were "uncomfortable at best and dangerous at worst" and cautioned that the coronavirus pandemic is not solely to blame for current pressures on A&E services across the country.
NHS guidance states that 95% of patients attending accident and emergency departments should be admitted, transferred or discharged within four hours.
But Northumbria Healthcare NHS Foundation Trust fell behind that target in May, when just 84% of the 9,148 attendances at type 1 A&E departments were seen within four hours, according to figures from NHS Digital.
Type 1 departments are those which provide major emergency services – with full resuscitation equipment and 24-hour consultant-led care – and account for the majority of attendances nationally.
It means 16% of patients needing the most urgent care at Northumbria Healthcare waited too long to be seen last month, compared to 14% in April, and 8% in May 2021.
Including the 11,292 attendances at other accident and emergency departments, such as minor A&Es and those with single specialties, 92% of A&E patients were seen by the trust within the target time in May.
The King's Fund think tank warned patients were waiting longer in A&E departments for a "wide range" of reasons, including rising A&E attendances and emergency admissions to hospital, fewer hospital beds being available and staffing strains.
"The four-hour A&E waiting time standard is one of the most high-profile indicators of how the NHS is performing," it said in a recent report.
"The sustained declines in performance against this waiting time standard place a significant toll on patients and staff alike and are a clear indication of the pressures the wider health and care system is under."
The 95% standard has not been met across the NHS in England since July 2015 – and last month, just 73% of A&E attendances were admitted transferred or discharged within four hours, compared to 84% in May 2021 and 87% in May 2019.
Performance was worse in type 1 departments, where just 60% of patients were seen on time in May, down significantly from 76% during the same month last year, and 79% before the coronavirus pandemic.