Rishi Sunak has insisted his plan to expand free childcare will go ahead despite facing "practical issues" in the run up to the rollout.
There have been reports the flagship policy, due to start in April, is in jeopardy because of delays in allocating funding, staff shortages and issues with the IT system behind the scheme.
Asked about the problems, Mr Sunak told broadcasters: "Many families have been able to sign up and it's all working fine, but there are some practical issues that certain families are facing.
"I just want to reassure all of those people that those issues are being resolved as we speak, all of those families will get the childcare that they are eligible for."
He said the free provision will be accessible "this spring" - calling it the "biggest expansion of childcare in our country's history".
Chancellor Jeremy Hunt announced the reforms for England in the budget last March to help parents with the soaring cost of childcare.
Under the plans, working parents of two-year-olds will be able to access 15 hours of free childcare from April. This will be extended to working parents of all children older than nine months from September.
From September 2025, working parents of children under five will be entitled to 30 hours' free childcare per week.
But nurseries have not yet been informed how much they will be paid for each of the places on offer, with many warning parents they will therefore not be able to immediately honour the government's free hours pledge, according to The Times.
There are also doubts about the extension due in September because of a lack of nursery staff.
The newspaper reported thousands of families will have to re-enter details into the HMRC IT system in March or risk delays in receiving payments.
The Department for Education (DfE) also initially miscalculated the cost of the scheme, resulting in delays in childcare providers finding out from councils how much funding they will get, the paper said.
The Times quoted unnamed Whitehall sources as saying "the strategy is flashing red all over the board" and "September is going to be an absolute s*** show".
Childcare providers have long warned the expansion of free provision could be undermined by capacity issues.
'Pre-existing feature' impacting parents
The DfE has acknowledged some parents might be impacted by IT issues.
A spokesperson said: "We are pleased that thousands of parents have already applied for the expansion starting in April.
"However, a pre-existing feature in the system, where parents reconfirm their eligibility every three months, is impacting a minority of parents when combined with a small number of providers who are asking for codes much earlier than April.
"Parents who can't reconfirm online until the second half of February or March will therefore automatically receive a letter with a code from HMRC before the middle of February, without needing to take any action."
Tory promise 'lies in tatters'
Labour hit out at the Conservatives' record on education and childcare, saying families are "paying the price" for a "disastrous failure".
Shadow education secretary Bridget Phillipson said: "First the chaos of crumbling concrete buildings, then the botched budgets for our schools, now the disastrous failure on delivering childcare commitments, with families paying the price.
"Funded hours are no good if families can't access them - the Conservatives' promise to parents now lies in tatters because there was no plan behind the pledge in last year's budget statement.
"Only Labour has a plan to transform our early years system and deliver the modern childcare system that gives parents choices and children the best start in life."
Labour is considering plans to create thousands of nursery places within existing primary schools and has commissioned former Ofsted head Sir David Bell to help find new ways to increase levels of childcare provision.
The issue could become a key battleground at this year's general election, expected in the second half of this year.
The UK has some of the highest childcare costs in the world, and experts have warned the most disadvantaged children are at risk of missing out due to the nationwide shortage of provision.
The number of registered childcare providers in England fell by 20,000 from 2015 to 2022, according to data from Ofsted.