More than one in three GP out-of-hours services do not satisfy national guidelines for assessing patients, a report suggests.
And research showed the proportion of calls prioritised as "urgent" in out-of-hours care varied from under 4% to more than 32% between services, "raising questions" over consistency.
In the most comprehensive review of NHS emergency and urgent care services, the Healthcare Commission found wide variations in how patients across England are treated.
People are confused over where to seek help, face unnecessary delays on being admitted to hospital, and some out-of-hours care is poor, it said.
More than one in three out-of-hours services cannot demonstrate that they meet national requirements on assessing patients within a set time-frame.
The review also found only 44% of out-of-hours GP services had arrangements in place to divert calls made to surgeries when they were closed.
Anna Walker, chief executive of the Healthcare Commission, said: "How an area deals with emergency and urgent care is absolutely a cornerstone of 21st-century care. There remains issues for patients about access to these services.
"Where we have found weaker performance, it tends to be out-of-hours services not working as well, more difficult access to GPs … and services that are less good for vulnerable groups, such as those with disabilities and long-term conditions."