Some out-of-hours services at weekends or evenings are failing to effectively prioritise which cases are the most urgent, a Department of Health study says.
The research raised fears that a lack of "safe and effective" systems for prioritising calls could mean urgent cases are being missed.
It also pointed to a lack of consistency, claiming there is a "wide variation" in which patients are defined as needing urgent care.
The study, which covers nearly 100 PCTs in England, was commissioned by the DH and was funded by PCTs.
It covered Christmas week last year when demand on services was high, but was no more than expected and "followed a predictable pattern".
Researchers found most services were failing to fully meet one or more key standards on out-of-hours care laid down by the NHS.
Performance in out-of-hours care is sometimes poor because of variations in staff ability and the way individuals' practice, which needs watching more closely, they said.
"The service performs well when some staff are on duty and less well when others are working," according to the study from the Primary Care Foundation.
More than one in 10 urgent cases are not assessed clinically within the required 20 minutes, but the figure varies from around 40% assessed within 20 minutes to almost 100% assessed within the timeframe.
Almost a quarter (24%) of less urgent cases are not assessed within the required 60 minutes and there is widespread variation.
The Department of Health said out-of-hours providers "have to demonstrate 100% compliance" with these standards. Where the average performance is more than 10% below the requirement, services are regarded as "non-compliant".