Ed Miliband has attacked the "snobbery" that suggests only an academic education is worthwhile, insisting that the UK must give more respect and value to vocational learning and apprenticeships.
The Labour leader called for action to open up the "closed circles" in elite professions and make it easier for disadvantaged children to go to university.
He accused the government of taking "backward steps" on social mobility by allowing inequality to grow and denying bright youngsters from poor backgrounds the chance to succeed.
"Tackling social mobility is a huge mountain to climb and the last Labour government took some important steps," Miliband told a Sutton Trust conference in London.
"But now Britain is sliding backwards. This government seems to think we can let those at the top take whatever rewards they think fit and somehow everyone else can just play catch up. You can't claim the mantle of social mobility and then make your priority, cutting taxes for millionaires while raising taxes for everyone else."
Miliband stressed allowing young people from poorer backgrounds to reach university is crucial to improving the situation.
"I know how much more there is to be done to open those closed circles of Britain. Not just to the universities, but to journalism, law, finance - and politics too," he is to say.
But he will also insist that there must be options for people who choose not to go to university.
"The debate has been too narrowly focused," Miliband said.
"We should reject the snobbery that assumes the only route to social mobility runs through university - as if there is only one pathway to success."
Miliband said a Labour government would include a requirement for companies bidding for major government contracts to offer apprenticeships.
But Deputy Prime Minister Nick Clegg will dismiss Miliband's criticism in a speech to the same conference on Tuesday.
He will insist that social mobility is the "impulse" that lies behind the government's education reforms, including the pupil premium.
And he will promise to address the "corrosive" rift in Britain's education system, citing new government data showing children educated privately are three times more likely than state pupils to attain top A-Level grades.
"Education is critical to our hopes of a fairer society," he will say.
"Right now there is a great rift in our education system between our best schools, most of which are private, and the schools ordinary families rely on. That is corrosive for our society and damaging to our economy.
"I don't for a moment denigrate the decision of any parent to do their best for their child, and to choose the best school for them. Indeed, that aspiration on behalf of children is one of the most precious ingredients of parenthood."
The Liberal Democrat leader will go on: "But we do need to ensure that our school system as a whole promotes fairness and mobility, that heals the rift in opportunities.
"We are committed to narrowing the gap in our school system - state and private - and ensuring that all children are given the chance to rise. The way to do that is to make the state education system better - to level up - and ensure that anyone can get ahead."
He is expected to reaffirm the coalition's drive on reforming the pre-16 curriculum, plus improve teacher and school quality.