The Pre-school Learning Alliance survey found that just 44 per cent plan to deliver the scheme, which is rolled out nationwide this September, compared with the 95 per cent that offer the existing 15 hours of free entitlement.
A chief concern raised is that the funding on offer by the government to deliver the extra hours is not enough. Nearly six out of 10 (58.3 per cent) respondents said the funding on offer does not cover the cost of delivery and a similar proportion (60.8 per cent) believe the flagship government policy will have a negative financial impact on their business, according to the 1,332 providers surveyed.
Meanwhile, a quarter (25.3 per cent) believe it is likely that if they offered 30 hours of free childcare they would be forced to close. Among those with financial concerns around the initiative, more than half (53.1 per cent) plan to pass the shortfall on to families through a fee hike for additional hours.
Just under half (47.9 per cent) said they would be forced to charge for goods and services that were previously provided for free, while four out of 10 (40 per cent) plan to restrict the days and times when funded places can be accessed.
Pre-school Learning Alliance chief executive Neil Leitch said the survey findings put the government's pre-election manifesto pledge to double the existing free entitlement "at risk" unless funding concerns are addressed.
"With so few providers currently committed to delivering the 30 hours, and so many forced to consider limiting places, raising fees or introducing extra charges in order to remain sustainable if they do offer it, many parents expecting easy access to a 'free' 30-hours place in September are likely to be disappointed," he said.
"The fact is that the 30 hours offer cannot succeed without adequate investment. Neither parents nor providers should be expected to pay for a promise that government chose to make."
Figures published by the Department for Education in February also highlighted concerns among providers, with 10,000 fewer planning to offer the 30-hour entitlement than offer the current 15 hours.
Last month, Chancellor Philip Hammond's 2017 Budget was criticised by the childcare sector for failing to include additional money for the extra entitlement.
However, the DfE has said that the eight pilot council areas already delivering the 30-hour entitlement are exceeding its ambitions. It said that 5,400 places have been allocated, surpassing the 5,000 target it set last year.
Early years minister Caroline Dineage said: "I have heard so many stories from parents whose lives have been improved by access to 30 hours, taking away the financial stress of accessing high-quality childcare and offering them the flexibility their families need.
"It's thanks to the creativity and dedication of the professionals already delivering 30 hours that parents are getting this huge boost, backed by our record investment of £6bn per year by 2020.
"The hard work of nurseries, pre-schools and childminders, as well as the eight early implementer councils, is opening doors for parents who previously felt like they were working just to pay for childcare."
But Purnima Tanuku, chief executive of National Day Nurseries Association, said: "All but one of these pilots were very small and well supported. They don't represent mainstream childcare and will not be able to address the general day-to-day issues which will arise from offering 30 hours of funded childcare.
"Most funding rates now being announced by local authorities will still not cover costs for the majority of nurseries."
Last month, the owner of one of the nursery chains taking part in the York pilot said the £4 an hour being offered to take part did not cover the cost of "the basics of high-quality childcare".