Fourteen hospitals have been chosen to pilot a new way to measure performance, which could pave the way for the end of the four-hour A&E target in England.
But the move is controversial, with some seeing it as an attempt to move the goalposts because the target has been missed for more than three years.
And the Royal College of Emergency Medicine has been critical of the plans, calling them "disappointing".
The trusts chosen include a mix of rural and urban sites and top and bottom performers.
They are: • Cambridge University Hospitals • Chelsea and Westminster Hospital • Frimley Health • Imperial College Healthcare • Kettering General Hospital • Luton and Dunstable University Hospital • Mid Yorkshire Hospitals • North Tees and Hartlepool • Nottingham University Hospitals • Plymouth Hospitals • Poole Hospital • Portsmouth Hospitals • Rotherham • West Suffolk
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Instead of aiming to see and treat virtually all A&E patients in four hours, the sickest patients will be prioritised for quick treatment.
NHS England has said it wants patients who come in with heart attacks, acute asthma, sepsis and stroke to have their care started within an hour.
All patients will be expected to be assessed by a key decision-maker, such as a senior nurse, within 15 to 30 minutes.
Average waits will also be monitored.
The pilots will start in the coming months with a view to introducing the new measures from next spring.
The Royal College of Emergency Medicine has warned that the plans could lead to "crowding and delays in assessments, antibiotics and pain relief".
Its president, Taj Hassan, even said it could even see the return of the "dark days" when patients spent 48 hours in hospital corridors.
The four-hour target, which expects 95% of patients to be seen in time, was introduced in 2004 and has not been met since July 2015.
When it announced the move, NHS England said the current target seemed to be distorting priorities.
It pointed out that large numbers of patients were admitted into hospital just before the four-hour mark, which has the effect of "stopping the clock".
NHS England said hospitals appeared to be motivated by the target rather than doing what was best for the patient.
Changes are also being made to other targets, including those covering cancer care and planned operations, such as knee and hip replacements.
No announcement has been made yet about which areas will trial those.
NHS England said the testing phase would be carefully evaluated.
"The information we gather through field testing and engagement will inform final recommendations ahead of full implementation beginning spring 2020."