Nick Clegg "gave up the right" to launch a social mobility strategy when he "betrayed a generation of young people", deputy Labour leader Harriet Harman has said.
Ms Harman told MPs that mobility now meant a "bus down to the job centre" for many young people.
The Government's wide-ranging strategy seeks to improve the life chances of children born into low and middle-income families.
Summoning the Deputy Prime Minister to the Commons for an urgent statement, Ms Harman told him: "I'm afraid you gave up the right to pontificate on social mobility when you abolished Educational Maintenance Allowance (EMA), trebled tuition fees and betrayed a generation of young people.
"When I heard the Deputy Prime Minister was going to launch a commission on social mobility, I thought it was April Fool's Day. In just 10 months, this Tory-led government has launched an assault on the opportunities for young people, especially the poorest."
Mr Clegg told MPs that while the Government's most urgent task was to sort out the nation's finances, its "overriding mission" was to take "real steps towards a fairer society".
He said: "For us a fair society is an open society - one in which everyone is free to flourish and rise regardless of the circumstances of their birth. That is why the promotion of social mobility is the principal objective of the coalition Government's social policy."
Mr Clegg defended the coalition's policies on EMA and tuition fees.
He told her: "Increased public spending doesn't in and of itself increase opportunity and social mobility. That is the serious question which I hoped you would engage in.
"There is nothing just, there is nothing that will help social mobility, in saddling our children and our grandchildren with this generation's debts."