GP out of hours services are costing more than three times more per patient in some parts of England than in others.
The research organisation Primary Care Foundation has found out of hours provision costs less than £3.50 per head in the cheapest areas, and more than £12 in the most expensive.
The Department of Health-backed study is the first to benchmark primary care out of hours services, taking data from more than half of all English primary care trusts.
The foundation said costs tended to be higher in less densely populated areas but there were still wide variations between PCTs in areas with similar population densities.
In PCTs with fewer than five people per hectare, cost per head ranged from less than £4 to more than £12.
Most variation is due to individual contractual arrangements set up by PCTs, the foundation said. This includes differences in types of service provided, such as requirements for more primary care centres to be open at particular times in some areas.
There was also wide variation in cost per case, from £30 in some areas to £100 in others.
The study raises concerns over the reliability of call handling services, with huge variations in the numbers of cases marked as urgent by staff.
In some areas, 5 per cent are marked as urgent, whereas in others the figure is more than 40 per cent.
Some services were providing telephone advice to more than two thirds of patients, with others offering it to less than a quarter.
But most services were complying or partially complying with national standards for timeliness.
No single type of organisation, whether commercial or PCT based, performed better or worse than the others.
Around 100 organisations provide out of hours services, ranging from GP co-operatives to commercial organisations.
A small number of GP practices still provide their own out of hours cover.
Primary Care Foundation director Henry Clay said: “PCTs and providers need to sit down and work out what this means in their own locality, [taking into account] commissioning decisions and geography.”