The Primary Care Foundation studied GPs working in emergency wards and estimates that half of all A&E clinics now employ primary care staff to treat patients.
But the DoH-commissioned study found little evidence this drives down NHS costs and that fewer cases can be treated by GPs than managers assume.
'We found the proportion that could be classified as primary care cases was between 10 and 30 per cent,' the report says. 'This contrasts with widespread assumptions that up to 60 per cent of (A&E) patients could be diverted to GPs or primary care nurses.
'Adding primary care practitioners may create short-term savings for commissioners but, without commensurate savings being made or other benefits being realised, there is no saving to the NHS or taxpayer.'
The report found the most common model in England involves a GP service alongside an emergency ward. GPs and primary care staff can also act as filters, screening attendees or diverting them elsewhere.
The study found that patients referred back to their GP practice from A&E were less likely to attend, and pre-assessments by triage nurses also led to 'a higher risk that an urgent case might be missed'.
The report comes after NHS London announced plans to close emergency wards in the hope that up to 60 per cent of A&E attendances can be seen by GPs in polysystems and urgent care centres.
Dr David Carson, joint director of the Primary Care Foundation, said it was 'vital to get this service right'.
'Alongside general practice, (A&E) is the front door of the NHS. Patients know who their GP is and where the nearest emergency department is.
'We were surprised to find there was no evidence that providing primary care in emergency departments could tackle rising costs or help to avoid unnecessary admissions.'
The College of Emergency Medicine backed integration of emergency and primary care departments, despite acknowledging that previous assumptions about such projects were 'flawed'.