Under plans set out in the party's manifesto earlier this month, Labour wants to extend 30-hour childcare to more children by eliminating means testing for two-year-olds and no longer restricting provision for three- and four-year-olds to children whose parents are working.
The party's shadow education secretary Angela Rayner today said the policy will cost £4.8bn a year, as well as £2.7bn of capital spending to create more childcare places and train staff.
Meanwhile £500m will be set aside to protect children's centres from further cuts.
Currently, all three- and four-year-olds are eligible for 15 hours of free early education each week, which will be extended to 30 hours for working parents from September. About 40 per cent of two-year-olds are currently entitled to 15 hours of free childcare a week.
Rayner said the Conservatives' 2015 promise to provide parents 30 free hours of childcare a week has "unravelled", as they have failed to give the policy the funding it needs.
"Too many parents have been let down, unable to go back to work due to the cost of childcare," she said.
"Unlike the Conservatives, Labour will properly fund childcare to help parents get back into work and ensure all children, no matter what their background, have access to the high-quality childcare they deserve.
"The Conservatives are failing to deliver on early years education, there are now over 1,240 fewer designated Sure Start children's centres than when the Conservatives entered government.
"It is clear Theresa May and the Conservatives can't be trusted with our children's futures."
However, during an interview on BBC Radio Four's Women's Hour today, party leader Jeremy Corbyn was repeatedly unable to provide a figure for how much the plans will cost.
"It will obviously cost a lot to do," Corbyn said.
"If we don't invest in our children and we don't invest in their future they do less well in primary school, less well in secondary school and less well in the future."
Neil Leitch, chief executive of the Pre-school Learning Alliance, said: "Labour is quite right to state that every child, regardless of background, should have the best start in life, and we are pleased to see the early years placed so prominently on Labour's political agenda.
"That said, as we warned when Labour's manifesto was announced, unless this policy is adequately funded then it simply won't work.
"Today's announcement details that their plans to rollout universal childcare to all two- to four-year-olds would cost an extra £5bn per year by the end of the parliament, however if these costings are based on the current, inadequate early years funding rates, then this pledge will inevitably be underfunded."
Purnima Tanuku, chief executive of National Day Nurseries Association said Labour's promise of free universal childcare from aged two is "an ambitious pledge which would help millions of families across the UK".
"Any realistic pledge to increase ‘free' childcare must be backed up by a fully costed workforce strategy with a commitment to improve salaries above the National Living Wage and make childcare an attractive career," she said.
"Nurseries will not be able to meet Labour's pledge to raise the minimum wage to £10 by 2020 unless there is substantial investment to pay for increased payrolls.
"These proposals must also have the full backing of the early years sector, which must be consulted at every step of the way to make sure the plans are deliverable."