Nurseries and other childcare providers say plans to allow more children for each member of staff in England will not cut costs for parents.
Most nurseries and pre-schools were opposed and would not increase numbers, the Early Years Alliance charity said.
One mother, whose two-year-old has severe allergies, told BBC News she did not want staff stretched further.
The government said it would consult on whether to allow each member of staff in England to look after more children.
Currently, in a nursery in England, one adult can look after four two-year-olds.
The government is to ask parents and childcare providers' views on whether to do the same in England.
But a snapshot of views gathered by the Early Years Alliance, seen exclusively by BBC News, suggests strong opposition.
Of the more than 5,800 nurseries and pre-schools that responded, 4,309 were strongly opposed, with a further 398 against the plans to some degree.
Childminders were more divided, with just over half of the 2,146 who responded against the plans.
Will Quince, Minister for Children and Families, has visited Scotland - where education is devolved - and is also looking at how other European countries operate before consulting the public on changes.
Ashlie Walker has four children but the one she worries most about is Reggie.
Reggie, two, has severe allergic reactions that can be life threatening and has already ended up in hospital several times as a result.
With a new baby to care for, Ashlie has to trust nursery staff to look after Reggie a couple of days a week.
She is anxious about them being stretched any further by having more children to look after.
"That's too much for them," Ashlie says.
"It's piling too much pressure on them. It's not fair to ask that of them."
The parents of a toddler who died last year after a medical emergency at nursery have started a petition against the changes, which has already reached 59,000 signatures.
For most, formal childcare costs are highest before their child's second birthday, when the number of children each staff member can look after is lower.
Allowing more over-twos would cut these costs, the government has said.
But nurseries say they are facing rising costs from increases to the National Living Wage and National Insurance, as well as energy and food prices.
Almost a quarter of families with a household income of between £20,000 and £30,000 a year spend more than £100 a week on childcare for one to two- year-olds, according to new analysis by the Institute for Fiscal Studies (IFS).
The IFS says this amounts to more than 17% of their pre-tax income.
It warns that blanket measures, such as relaxing staff to child ratios, would mainly benefit high-earning parents of younger children, in the most expensive parts of the country.
There is also concern about the pandemic's impact on young children.
An Education Endowment Foundation study found fewer Reception pupils in England were fully ready for school this year
Only 59% of four- and five-year-olds were at the expected level for key markers such as social skills, speaking and understanding words or numbers, and physical development - down from 72% in 2019.
At Little Toes Nursery, in Nottingham, owner Rachel Buxton says lockdown babies need extra help to develop and there are rising wellbeing concerns
"Some of our children have got speech delay, so we're seeing more speech-and-language referrals," she says.
"They've not had the opportunity to socialise, they're not just alone at home with a parent anymore and they need help from an adult with those social skills."
So even if ratios are relaxed, Rachel has no plans to ask staff to look after more children.
She also fears a change in the rules could be used as a reason to hold down the hourly rate of funding for the free hours the government offers to parents.
Early Years Alliance chief executive Neil Leitch said the funding had already fallen behind costs and, amid a recruitment crisis, it was "ridiculous" to imagine any savings would be passed on to parents.
"When I talk to people who are leaving the sector, they say they are exhausted and undervalued, so they are completely opposed to the proposal they take on more and more children, give these children less and less time," he added.
A government spokesman said more than £3.5bn had been spent in each of the past three years to support free hours of childcare, including the 30 hours a week for working parents of three- and four-year-olds.
"Ministers have been clear that supporting families with access to childcare and early education is a priority," he said.
"We are working with colleagues all over government to look for ways to improve the cost, choice and availability of childcare places."
Labour says instead of altering the number of children for each staff member, it would quadruple the funding targeted at the poorest early years children.