The Nuffield Trust health think tank said delays to care are "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 A&E departments should be admitted, transferred or discharged within four hours.
But Milton Keynes University Hospital NHS Foundation Trust fell behind that target in May, when just 72% of the 9,172 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 28% of patients seeking the most urgent care at Milton Keynes University Hospital waited too long to be seen last month, compared to 25% in April, and 16% in May 2021.
Including the 5,565 attendances at other accident and emergency departments, such as minor A&Es and those with single specialties, 82% of A&E patients were seen by the trust within the target time in May.
"The four-hour A&E waiting time standard is one of the most high-profile indicators of how the NHS is performing," it stated.
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.
Nuffield Trust fellow Jessica Morris said: “Performance against the four-hour target has deteriorated rapidly during the pandemic but it is clear waiting times for patients in A&E have been growing for the last decade.
“A decade ago, one in 20 people found themselves waiting longer than four hours to be either admitted, transferred, or discharged from major A&E but the most recent figures show this has jumped to four in 10 people. These waits are uncomfortable for patients at best and dangerous at worst."
She added that a lack of resources and staffing pressures are fuelling waiting times across the entire health service.