Nick Clegg has said £7bn will be spent over four years improving the education of England's poorest children.
It will provide 15 hours of free pre-school education a week for two-year-olds, a "pupil premium" for those at school and help to get to university.
The deputy PM said next week's spending round would see "difficult cuts" but would also invest in "fairness".
Labour's Andy Burnham suggested Mr Clegg was trying to appease his own MPs after a difficult week on tuition fees.
Deputy Prime Minister - and Lib Dem leader - Mr Clegg is under pressure to show that he is fulfilling his party's manifesto commitment on the "pupil premium", amid anger from some of his MPs at his U-turn on university tuition fees.
In a speech in Derbyshire, Mr Clegg said the £7bn "fairness premium" would support the poorest young people from the age of two to the age of 20.
He said that although the government faced a "hard road" with next week's spending round - which is expected to see departmental cuts of between 25% and 40% - it would "not compromise on a better future for the poorest children".
He suggested the £7bn commitment would mean "even tougher choices elsewhere" but added: "The right thing to do is improve the life chances of the poorest by investing in a fairness premium even as we cut spending in other areas.
"The right thing to do is to invest in the future, even if it makes it harder today."
By the end of the four-year Spending Review period, money allocated to the pupil premium would rise to £2.5bn a year, he said.
While details were still being worked out, he said if that were to be applied to the 1.2 million children eligible for free school meals, it would amount to about £2,500 funding per child per year.
The Lib Dems have suggested schools could choose how to spend the pupil premium to try to narrow success rates between their richest and poorest pupils - for example through extra one-to-one tuition or after-school clubs.
Mr Clegg said about £300m would go on the pre-school support and about £150m a year would be targeted at bright youngsters who might otherwise have been "deterred" from going to university.
"All disadvantaged two-year-olds" would be offered 15 hours a week of free nursery education, on top of those already available at ages three and four. It builds on pilot schemes introduced by Labour offering between 10 and 15 hours a week to 15% of the poorest two year olds. The Lib Dems say under their plans, 130,000 two-year-olds will benefit.
Details of how the "student premium" would work were still being ironed out, he said.
Questioned after the speech Mr Clegg dismissed suggestions he had raised it to calm his backbenchers' fears over tuition fees, adding: "I first wrote about the pupil premium a decade ago. I have been talking about this for years and years."
The £7bn falls short of the Lib Dems manifesto commitment - which pledged about £2.5bn a year from 2011 for the pupil premium - amounting to more than £10bn over the four-year period. The Conservatives had also committed to a pupil premium.
In the parties' coalition agreement they agreed to "fund a significant premium for disadvantaged pupils from outside the schools budget by reductions in spending elsewhere". The government has not confirmed how the £7bn will be found.
Shadow education secretary Andy Burnham told the BBC: "We've got to apply a very, very hard test to Mr Clegg's announcement today. It comes out of nowhere after a hard week for the Liberal Democrats.
"They're saying that this pupil premium is their flagship commitment from the coalition agreement. They promised it would be funded from outside of the schools budget. I will be holding him to that. If he doesn't demonstrate how this will be funded from outside of the school's budget then I'm afraid it will be another hollow promise."
And Chris Keates, head of the NASUWT teachers' union, dismissed the measure as a "sop" to Liberal Democrat back benchers which would "sink without trace" when wider spending cuts are announced next week.
The Institute for Fiscal Studies said, based on the government's consultation over the pupil premium, their researchers believed it would be "broadly progressive" - as it meant schools with more poor pupils would get extra funding.
But it warned that under the current plans, deprived schools in poorer areas would get a smaller percentage increase in funding than schools with a high number of deprived pupils in better off areas. They urged the government to make sure the same funding was attached to all eligible pupils, regardless of where they live.