Nick Clegg has hailed progress in opening up top careers to people from less privileged backgrounds after 100 companies backed a government scheme.
Barclays, Tesco and BP, as well as 10 top law practices are promising to advertise work experience places and pay interns wages or expenses.
The deputy prime minister said this marked a "culture shift".
But Labour said high youth unemployment made promises to increase social mobility "a bit rich".
Mr Clegg's announcement comes as research suggests that more than a third of graduate vacancies available this year are expected to be taken by people who have already worked for the firm while they were studying.
The deputy prime minister told the BBC: "If you feel you can't even get your foot in the door to take up a work experience place at a local company, or take up an internship, if you feel you can't even take that first step, then what chance do you feel you've got to actually work at that company later?
"It's not a small matter. It's quite a big deal to encourage these companies.
"I'm delighted that 100 really big companies employing over two million people have now agreed with my ideas on this.
"They are going to open their doors and make it much more based on what you know, rather who you know, to get that first foot on the first rung of the jobs ladder."
Some 115 businesses and other organisations have agreed to:
- Advertise work experience opportunities in schools and online rather than using informal contacts
- Make access to internships open and transparent
- Either pay interns a wage, reimburse their expenses or provide accommodation
- Ensure application forms do not "screen out" people on the basis of their education or ethnicity
- Visit schools and colleges to talk about their work and take part in mentoring schemes.
Among those backing the scheme are BAE Systems, Coca-Cola, Ernst & Young, Vodafone, Marks & Spencer and WPP.
Mr Clegg, who has admitted he benefited from informal contacts at an early stage of his career, is writing to 50 other companies to ask them to take part in the voluntary initiative.
'Bank of mum and dad'
He said the agreement built on other steps taken by the coalition to improve the life chances of those from disadvantaged backgrounds, such as expanding free education for two-year-olds and extra funding for pupils from poorer backgrounds.
Campaigners for fair access to internships welcomed the backing of the companies but said not enough was being done to ensure people could afford to take up the opportunities on offer.
"Employers don't need an opt-in scheme to have a legal duty to pay their interns," Ben Lyons, co-director of Intern Aware, said.
"A few pounds a day in expenses is not enough for anyone to live on without huge support from the bank of mum and dad.
"If Nick Clegg is truly committed to social mobility he will make sure that Revenue and Customs enforces the minimum wage for interns."
Asked if teenagers from poorer backgrounds could afford to take up unpaid placements, Mr Clegg told the BBC that some big companies were already paying interns the national minimum wage for "a proper day's work".
But schemes came in "different shapes and sizes" and, for some, it did not "make much sense giving them a full wage", as help with lunch, travel and accommodation costs was more appropriate.
Labour said government cuts were holding back the prospects of a generation of young people.
"It's a bit rich for Nick Clegg to talk about social mobility when youth unemployment is over a million," Jon Trickett, shadow Cabinet Office minister, said.
"This government's choices - abolishing the future jobs fund, trebling tuition fees and scrapping Education Maintenance Allowance - tell you all you need to know about their priorities."
And the National Union of Students called on the Lib Dems to get "their own house in order" by ensuring their MPs paid interns working in their offices a proper wage.
The NUS pointed out that a number of Lib Dem MPs are advertising for unpaid interns, offering lunch and travel expenses only.
"It is not enough to exploit loopholes in the law to avoid paying ambitious people," said NUS vice-president Dannie Grufferty.
"MPs of all parties must take a moral lead on this issue and commit to paying all their interns a living wage."