As many as 140,000 disadvantaged two-year-olds could have free nursery or childcare places under a scheme planned to be rolled out in England.
Plans to give 15 hours of free "early education" a week to all two-year-olds from poor homes were announced last year but details have now been set out.
A pilot scheme, first started under Labour, is due to be extended nationwide from September 2013.
Children's charities have welcomed the news but say more investment is needed.
At the moment, all three and four-year-olds are currently entitled to 15 hours of early education for 38 weeks a year.
This can be in nursery schools and classes, children's centres, day care nurseries, play groups, pre-schools and with accredited child minders.
Under the new plan, which is out for consultation, parents will be able to use their allocated time flexibly, between 07:00 and 19:00.
So for example, they could have seven-hour slots for two days a week, to make it easier for them to work.
'Balance work and home'
Ministers say access to early education improves the life chances of poor children by helping them develop and get ready for school.
Disadvantaged children are far less likely than others to do well in education.
Under the pilot, 20,000 two-year-olds received the free provision in a year.
Extending it to all two-year-olds in low income families would mean 140,000 stood to benefit every year, the government said.
Deputy Prime Minister Nick Clegg said: "I want us to give every child the best possible start - so free education for toddlers from the most disadvantaged homes will now be a right and not a privilege.
"Crucially the extra care will be flexible and easy to access. Parents across the country are bending over backwards to balance work and home. The coalition wants to help in whatever way we can."
Families where children would qualify for free school meals are those who would be entitled to the free care.
The Children's Minister Sarah Teather said: "Our priority is to increase social mobility by helping children from the poorest backgrounds in their earliest years.
"High quality early education is the key to making a difference early on in a child's life. It's crucial for their healthy development and means they're not falling behind before they have even started primary school."
The charity Save the Children welcomed confirmation of the nursery places, but warned that spending cuts could damage the government's ability to improve children's lives and social mobility.
Head of policy, Chris Wellings, said: "We welcome the places for deprived two-year-olds. A lot of evidence shows that high-quality education can give disadvantaged children a better early start.
"We also welcome pilot schemes which are giving individual support to parents to help them improve their home learning environment.
"But we are concerned that the government is also cutting money for early years. Early years and youth services will be cut by 20% in this Parliament. We need investment in this area if the government is going to deliver on its promises on social mobility."
The government has also published a breakdown of how many two-year-olds would qualify in different parts of England.
More than 20,000 would be eligible in London and a similar number in the north-west of England.
Nearly 17,000 in the West Midlands would qualify and a similar number in the South East. In Yorkshire and the Humber the figure is nearly 15,000 and in the North East it is nearly 8,000.
All 152 councils in England have been involved in the pilot schemes. Some are looking for reassurance that there will be funding for the new places as their grant from central government is being cut by about 25% over this parliament.
Councillor Nickie Aiken, from Westminster City Council, said: "Westminster City Council was involved in piloting this scheme and we welcome these proposals to give disadvantaged children a head-start in their education, to simplify the guidance around early years provision, and to focus on quality teaching and care to improve the outcomes for our young children.
"However, we need clarification from the government as to where these hundreds of extra nursery places are going to come from. Once the new proposals are fully rolled out, Westminster will need to provide an additional 600 free places to two-year-olds, and the practicalities of this need to be explained to local authorities."
The government is funding 15 trials in 18 local authorities to test approaches to expand free early education and look at best practice across the country.
The government says it has included an allocation for the scheme up to 2013 in its early intervention grant to councils and is consulting on how best to fund the free nursery places from then on.