Members of Urgent Health UK (UHUK) - a federation of not-for-profit uncscheduled primary and community care providers - reported 'considerable pressure' and intense workload over the Christmas period.
A poll carried out by the federation found that members were facing rising workload, from single-figure percentage increases up to 29% in one case compared with last year.
UHUK said recruiting GPs to fill shifts 'continues to be challenging' - all respondents to the poll reported that hiring GPs was harder than last year, despite support from the winter indemnity scheme.
NHS England doubled funding this winter for the scheme - which runs from October 2017 to April 2018 - to £10m. Some UHUK members said it had helped them bring in GPs, but others warned that the temporary support could mean recruitment fell off a cliff once the scheme elapsed.
UHUK chair Dr Simon Abrams: 'Pressure on health services has been intense this winter especially over the bank holidays. Urgent primary care providers, the out-of-hours services have worked hard to provide urgent GP care for patients. They make an important contribution to healthcare throughout the year but especially at this time, keeping patients out of hospital and supporting other urgent care services.'
Warnings over heavy workload in urgent care came after NHS England decided earlier this week to cancel all NHS elective care until the end of January to ease strain on struggling hospitals. Hospitals reported 91.7% of beds full in the week to 31 December - up from 90.9% the week before.
For December as a whole, bed occupancy across the NHS in England has averaged 93.1%, analysis by GPonline shows - far above the 85% target that experts consider safe.