One of the coalition's key reforms has been thrown into doubt after Nick Clegg signalled he did not support it.
The government wants to allow childminders to look after more youngsters in order to drive down the cost of childcare.
But a spokesman for the Deputy Prime Minister said he "remained to be convinced" that the ratios should be relaxed.
The policy, being championed by Tory education minister Liz Truss, emerged after months of wrangling between the coalition parties.
From September, the ratio for under-ones had been due to rise from three per adult to four.
Each adult would be able to look after six two-year-olds instead of four. But the ratio for three-year-olds would stay at eight or 13 children per adult, depending on whether a qualified graduate was present.
Truss has argued that the changes would lower childcare costs and allow professionals in the sector to be paid higher salaries.
But the proposals have run into fierce opposition from experts and unions.
Dropping them could have serious consequences for the wider childcare provision shake-up, which includes tax breaks worth £1,200 for families where both parents work.
Clegg's spokesman said: "The delivery of good quality affordable childcare is one of Nick Clegg's biggest priorities in government.
Clegg has doubts about nursery reforms
"He has looked very closely at proposals to increase the number of children each adult can look after - and at the very serious concerns raised by parents and childcare providers in the recent government consultation.
"Nick remains to be persuaded that this is the right thing to do for very young children.
"Or, crucially, to be persuaded that this would actually help families with high childcare costs.
"This continues to be discussed in government."
Shadow education secretary Stephen Twigg said: "David Cameron's childcare plans have descended into chaos.
"Labour has warned for months that increasing the number of children that nursery staff look after would threaten child safety and the quality of care.
"After intense opposition, including from parents, childcare staff and experts, the Government appear to be U-turning on ratios.
"Ministers need to come to Parliament to confirm that these proposals are being dropped."
A Downing Street spokesman said: "We are reforming the childcare system so that providers have more flexibility when they have highly qualified staff.
"Ratio changes, which are not compulsory, will allow providers to have the flexibility to increase pay for better qualified workers.
"We are clear that quality and safety are paramount - any changes to ratios will only be on the basis of increased qualification levels and will not be mandatory.
"The consultation was agreed government policy and we will respond to it in due course."