Researchers from the Cambridge University’s Centre for Health Services Research and the Primary Care Research Group at Peninsula Medical School used feedback from 567,000 respondents to the English General Practice Patient Survey.
They found that poorer access during normal working hours was linked to increased use of out-of-hours services.
Measures of poor access included ease of getting through on the phone, ability to see their preferred GP, ability to book a routine or urgent (within two working days) appointment and convenience of opening hours.
The authors, writing in BMJ’s Emergency Medicine Journal, estimated that out-of-hours demand could be cut if access during working hours were optimal.
Currently 7.5% of people report using out-of-hours care, but this could be cut to 6.7% - a relative reduction of 11%, the report's authors said.
Convenient opening hours was the strongest factor associated with out-of-hours use, they said, which was most true for patients in part-time employment or not in work.
Even when opening hours suited people in part-time work, they were more likely to attend out-of-hours services than those in full-time work or education.
The authors concluded that improving access during standard hours, with for example more day time appointments, could be more effective at reducing demand on expensive out-of-hours services than extending hours.
They said: 'We found strong evidence that difficulties in accessing in-hours primary care are associated with higher use of out-of-hours primary care in England. The findings can help further motivate the development of interventions to improve patient experience of in-hour access.
The findings could cast doubt over the government and NHS England’s key Challenge Fund programme which is funding pilots schemes focused on extending GP opening hours.
GP leaders have argued for more resources to improve existing services.