A flagship government childcare scheme is at risk of “falling apart” with parents struggling to access new free hours and nurseries in the dark about if they can afford to provide care, according to charities.

Parents’ groups have accused the government of planning the new free offering “on the back of a fag packet”, with thousands of “furious” parents struggling to sign up for the scheme, which starts in April.

Early years providers say the £4bn scheme announced in last spring’s budget is “ill-thought out” and that they still do not know how much money they will receive to provide new free places, leaving them unable to judge if they have the capacity and staffing they will need.

“Yet again it feels as though the planning for childcare funding has been done on the back of a fag packet, leaving providers and parents to pick up the pieces,” said Joeli Brearley, the founder and CEO of the campaign group Pregnant Then Screwed.

“Investment has always been crucial, but so is consultation and delivery,” she said. “Without a robust strategy for all three, you’re just chucking cash into a black hole. I don’t think the government fully appreciates how furious some parents are.”

From April, eligible working parents of two-year-olds in England will be able to apply for 15 free hours of childcare a week during term time, with a wider rollout providing 30 hours to all eligible children from nine months to five years by September 2025.

But some parents already using the tax-free childcare (TFC) scheme, which gives eligible working parents who do not qualify for current free hours a 20% saving on fees, report being unable to get the new code they need in time to access April’s scheme.

Parents using TFC must reconfirm that they are eligible every three months and can reconfirm four weeks before the end of the eligibility period. Some who reconfirmed before the new scheme’s 2 January opening date for applications are unable to reconfirm again before March and are concerned about not receiving their code in time.

A flash survey of Pregnant Then Screwed followers attracted more than 4,500 responses in 29 hours. It found that only 10% of eligible respondents had got the required code, 69% said they had been unable to apply for it yet and 17% said they were unable to apply because they didn’t understand the system.

There was further consternation last week when the UK’s largest childcare provider, Busy Bees, told parents they had to provide codes by 16 February. The organisation said it had sent out the letter before it became aware that some parents were facing a delay in receiving codes and promised to work with them to process funding as soon as codes were received.

A spokesperson said: “We are committed to ensuring all eligible parents are able to access the new funding from April.”

The Department for Education said there was no fault with the system and that it would share more information to alleviate concerns for parents reconfirming in March shortly.

A spokesperson said: “The childcare application system is working as intended, with thousands of parents applying for and receiving codes to access their new free childcare entitlements every day.

“We are working to ensure all parents can access their codes in time to use the new entitlements in April and confirm childcare places as soon as possible.”

Some parents have reacted with dismay to the difficulties they have faced in signing up for the new scheme. A recent comment from a parent on a Pregnant Then Screwed social media post said: “My brain is swimming in soup at all this chaos and confusion.” Another parent wrote that the situation was “an absolute shambles”.

Other had greater concerns. Laura, a parent of a three-year-old and a nine-month-old said her nursery in Northamptonshire had not only decided not to provide government-funded hours for two year-olds from April, but it was also pulling the 30 free hours of childcare currently available for three- and four-year-olds.

“They said [it was] because of the uncertainty about fees and the fact they were so underfunded,” she said. “It has been incredibly stressful, because with two kids that is a big financial impact, and we had actually planned our second child around the new free hours.”

Sarah Ronan, the director of the Early Education and Care Coalition (EECC) said the government had confirmed the hourly rate it gives to local authorities in November, but local authorities had until 31 March to tell providers, with the scheme starting on 1 April. Many were still in the dark, she said.

“Until providers know how much of the government funding they are going to actually get, how on Earth can they plan their provision and staffing,” she said. “This is an ill-thought out policy. We expected teething problems, but the fact that both parents and providers are confused and struggling does not bode well.”