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.