Nicola Sturgeon has said she "deeply regrets" that her government's free childcare pledge has been put on hold for a year because of coronavirus.
Ministers confirmed on Wednesday that plans to offer 1,140 hours of free early years care will not be introduced during the 2020-21 school year.
Ms Sturgeon said this was "inevitable" due to the impact of the pandemic.
Opposition parties said the delay was "a bitter disappointment" and would be "devastating for parents".
The first minister said ministers would get the policy "back on track and deliver it as quickly as possible", saying she was "just as committed today as I was pre-Covid to fulfilling this commitment in full".
Ms Sturgeon announced proposals to almost double the amount of free early years learning youngsters receive - currently 600 hours - in 2014, shortly after taking over as SNP leader. Increasing free childcare had also been part of the Yes campaign's platform in that year's independence referendum.
The move would have benefitted all three and four-year-olds as well as some two-year-olds, but the planned implementation date was pushed back from August 2020 due to the coronavirus pandemic.
Early years minister Maree Todd has now written to councils to say it "would not be feasible" to have the system in place across all councils in the coming academic year, "or while the coronavirus public health measures remain in place".
A revised date for implementing the policy will be jointly agreed by the government and councils, and Ms Todd said an "initial assessment of readiness" would be carried out in December 2020.
Speaking at her daily briefing, Ms Sturgeon said she wanted parents and children to get the "fullest benefit" of the policy as soon as possible.
She said: "The expansion of childcare is one of the commitments of my government in this term of parliament that mattered most to me because it is so transformational in terms of the opportunities and the attainment of young people later in education.
"It also is a massive financial benefit to parents, so I deeply regret that it has to be slightly delayed because it was on track to be delivered - a massive policy, so that was no mean feat in itself.
"But I hope people will understand it is just inevitable."
The Scottish Conservatives said parents would be "furious" about the "staggering delay".
Education spokesman Jamie Greene said: "Parents understood why there was a delay to the additional hours, but an indefinite postponement of over a year is truly unacceptable and will devastate our economy and the nursery sector.
"No-one is fooled, the SNP over-promised to parents and now parents will be paying the price, either from their pockets or their inability to work."
Scottish Labour MSP Iain Gray said the development "simply isn't good enough", adding it would "leave many wondering if they will have sufficient childcare to allow them to return to work".
He said: "The Scottish government should make the required resources available so that the 1,140 hours they promised can be delivered as soon as possible. This failure is bad for children, bad for parents and bad for the economy."
The Scottish Greens said many parents and particularly women could be excluded from the workforce, with MSP Alison Johnstone saying: "Many families have planned their lives around the expectation that their child would be attending nursery from August, and the consequences of this now not happening will be stark."
The Lib Dems also called the delay a "bitter disappointment", with MSP Beatrice Wishart adding: "The Scottish government needs to change its approach and put a greater effort into getting the provision increased."