Nicola Sturgeon has said her government's plans to almost double free childcare by 2020 are "on track" in the face of opposition concerns about funding, staff and timing.
Scottish Labour leader Richard Leonard and his Liberal Democrat counterpart Willie Rennie both highlighted an Accounts Commission report which warned about "significant risks" to the policy.
The government has pledged to increase paid-for nursery places from the current 600 hours to 1,140 for three and four-year-olds, and eligible two-year-olds, by August 2020, equivalent to about 30 hours a week in term time.
Speaking at First Minister's Questions, Mr Leonard highlighted elements of the report including a £160 million yearly revenue funding gap between government and council estimates, a similar £290 million gap on cash needed to upgrade buildings and a lack of progress on securing 12,000 extra staff councils claim to need.
He said: "This government rightly made childcare its flagship policy but as it stands there is not enough money, not enough staff, not enough buildings to keep that promise."
Mr Leonard referred to a recent report from campaigners which found council nurseries offering the free hours are not open long enough for working parents.
He added: "No one can believe because your childcare promises for the future because your policies in the present are failing.
"Local councils say it, parents say it, Audit Scotland says it. When will you start to listen?"
Mr Rennie said he backed the policy but is concerned that the Scottish Government will "not be able to deliver it".
"I do agree with [Nicola Sturgeon's] ambition, I really want this to work but if everything is ok why are so many organisations speaking out?" he said.
"At the current rate of progress it will take another 20 years to recruit the staff needed and it will take 45 years before places are available everywhere during the school holidays.
"Three years after the First Minister made this promise, why is the government so far behind?"
Ms Sturgeon replied: "We're not far behind and I have to say some of the claims that Willie Rennie has just made there are ridiculous and will be seen to be ridiculous in a few years time when we have delivered this commitment just as we delivered the 600 hours when many people across this chamber were sceptical that we would do so."
She added: "We are on track to deliver this expansion."
The First Minister said one of the reasons behind increasing the hours is to improve flexibilty for parents.
She said it is not unusual to have initial disagreements on the money required but plans to have agreement with council umbrella body Cosla by the end of April.
She added: "This is one of the key policies of this government...it is one that we are determined to deliver and determined to put the funding and the planning in place to make sure that we can and we do."