Only a quarter of GPs said extended hours funding covers their costs. Many ran the service at a loss because not doing it would be even more costly.
One GP said providing extended hours cost his practice £22,000, but not opening longer would cost £28,000.
The extra hours, as predicted by the GPC, are creating stress within practices. The poll, of 301 GPs, found that more than a quarter felt relationships between partners, managers and staff had become strained as a result of extended hours.
It found evening and weekend clinics are generally well used, with 77% of GPs reporting that over three quarters of appointments are used.
But one in 10 GPs said less than a quarter of their extended hours appointments were used.
Respondents also reported more patients missing appointments, especially on weekends.
Just 17% of GPs said longer opening had improved the care at their practice.
GPC negotiator Dr Beth McCarron-Nash said: ‘I'd like to see the government read the results of your survey and decide how to make access better in a way that can really meet the needs of patients.
‘The funding for the DES (directed enhanced service) was always inadequate.'
A DoH spokeswoman said 1.1 million people had expressed a desire for extended hours in the latest GP patient survey.
‘Making these additional slots available to patients reduces pressures on general practice during other routine opening times,' she said.
‘Additional opening is therefore a core part of what patients want their practices to do and not simply an additional activity to be funded at premium cost.'
- Read this week's GP dated 21 August for the full version of this story and a two-page analysis of the issue.
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