Extending free childcare to 260,000 toddlers from low income families will help close the gap between rich and poor, Nick Clegg has said.
The Deputy Prime Minister set out details of an extension of the scheme for two-year-old children to cover the 40% of families "most feeling the squeeze".
The Liberal Democrat leader said his party wanted to extend free support to all two-year-olds but acknowledged there were "limited resources" to fund the policy.
Households in England which qualify for free school meals are already eligible for 15 hours a week of free childcare and the policy will now be extended to families earning less than £16,190 per year who receive working tax credits as part of a £760 million package.
Adopted children, those in care and youngsters with a disability or special educational needs will also benefit from the changes to be brought in next September, Mr Clegg said.
Speaking at a West London nursery the Deputy Prime Minister said: "I know that some of you will be thinking 'why not give this free support to every two-year-old? Why not help every family?'
"And it is certainly the Liberal Democrats' long-term ambition to extend free support to all two-year-olds. But the fact is that at a time of limited resources you've got to start somewhere.
"And for me, it's better for us to start with those children who can benefit most from high quality early years education, but who too often miss out.
"All the evidence shows that if you take two young children - hanging up their coats next to each other on the first day of school - the poorer child will already be behind their better-off classmate.
"And if we don't step in to help these children, that gap just keeps getting bigger. We're talking about a child's journey through life already being mapped out for them before they've even set foot in a classroom.
"Well-off children are more likely to become well-off adults. Poorer children are more likely to stay poor.
"And not only do these children suffer, the whole class suffers as teachers have to focus more of their efforts on children frustrated and left behind through no fault of their own."
Some 130,000 two-year-olds (20%) are currently eligible for free childcare, based on whether their family qualifies for free school meals or whether they are cared for by a local authority.
Shadow education secretary Stephen Twigg said the Government was failing to deliver the extra nursery places needed for the measures it had already announced.
He said: " David Cameron has created a cost of living crisis for hard working families. Whilst parents are feeling the squeeze of childcare, the Prime Minister has presided over reduced support, rising costs and fewer places.
"This out of touch government is failing to deliver the additional capacity needed to meet the commitments it has made for new places for two-year-olds.
"All this from a Prime Minister who promised the most family-friendly government in Europe."
Anand Shukla, chief executive of the Family and Childcare Trust, voiced fears that nursery closures could impede the delivery of free childcare.
"We are concerned that loss of nursery provision in children's centres is impacting on local authorities' ability to find sufficient places for the offer," he said.
New research by the Family and Childcare Trust - to be published this month - indicates that a minimum of 108 nurseries across England have closed or were never commissioned as they were supposed to be, he added.
"Cost savings have driven nursery closures and this approach reduces capacity in the system which will damage Government hopes of meeting its targets for provision of childcare for two-year-olds," he said.
The Pre-school Learning Alliance educational charity welcomed the announcement but said the Government plans would fail if they were not properly funded.
Neil Leitch, its chief executive, said: "This is a tremendous initiative that will help to support young children who statistically run the risk of being marginalised throughout their entire life."
But he added: "Our fear is that should this well-intentioned initiative be grossly underfunded, the Deputy Prime Minister will not achieve the brighter start in life for these children that he wants."
Mr Clegg also defended plans to end the "Edwardian" rules on parental leave which had personally frustrated him when his three children were born.
He said: " From day one in Government, we've worked with the belief that if modern families no longer fit the system, then it's the outdated system that needs to change.
"That's why, from next year, we're extending the right to request flexible working to every employee. So that the vital back-up team of grandparents, family members and friends who would love to do more to help you out now can.
"And, from 2015, if you're a new parent you'll also have greater freedom and flexibility to use and share leave during the maternity leave period in a way that works for you."
He added: " We are revolutionising parental leave so that it becomes... in effect it doesn't become maternity leave anymore, it's parental leave.
"You and your wife or partner will basically be able to chop and change and exchange the block of leave that presently is afforded to the mother, as you wish.
"I know plenty of women who feel they want to go back to work earlier than their partners or their husbands do. W e've got these Edwardian rules - I've experienced them myself three times, it's crazy. When a child is born you've got just a couple of weeks when in essence you are least relevant to the child. They're barely aware that you're there.
"I think it makes much more sense for dads who want to be involved in their child's upbringing, particularly if their wife or partner is working. I think (we should), wherever we can, help dads who want to get involved, particularly in those early months and years.
"All the evidence is overwhelming, it has a very profound bonding effect."