Nick Clegg has promised the government had changed the network of informal internships which he was "gobsmacked" to discover in Whitehall.
The deputy prime minister renewed his pledge to end Britain's "who you know" culture as he revealed that more than 100 companies have signed up to social mobility rules.
Supermarkets, banks, law firms and energy providers have committed to advertise work experience places openly rather than handing them out through contacts.
On a visit to the London office of software giant Microsoft, which is taking part in the initiative, he said: "When I came into government just over 18 months ago I was gobsmacked to find out there was a whole network of informal interns in Whitehall that's funded by the taxpayer, where it was all about who you know rather than what you know."
He added: "We have changed all that and it's part of a changing culture where we make access to work and opportunities to work fairer and fairer."
The issue has previously proved a source of tension in the coalition, with prime minister David Cameron openly contradicting the views of his Liberal Democrat deputy.
Last April Cameron insisted he was "very relaxed" about giving work experience to personal acquaintances, such as a neighbour who interned at his constituency office.
But Clegg shot back: "I'm not relaxed about this at all."
Today he announced that Barclays, HSBC and Santander, and retailers Asda, Tesco, Sainsbury's, Marks & Spencer and Morrisons had signed up to his scheme.
Other well-known companies taking part include Coca-Cola, Nestle, law firm Allen and Overy, and energy giants BP, Shell and E.ON.
The participants, which together employ more than two million people in Britain and have a turnover of over £500 billion, have agreed to:
:: Work with schools to encourage pupils' ambitions, for example through visits by staff and mentoring schemes;
:: Advertise work experience places in schools, online and in other public forums, rather than just distributing opportunities via informal contacts;
:: Make internships "open and transparent", with financial support so that young people from poorer backgrounds are not put off;
:: Recruit fairly, using application forms that do not allow candidates to be screened out because they went to the wrong school or come from a different ethnic group.
Clegg said: "I was helped by my family to find internships and get opportunities. That in a sense isn't going to stop over night, of course not.
"It's the most basic instinct for any parent to try and help their child.
"But I think for big companies and for instance government and government departments, we have all got a responsibility to society as a whole to make sure that the opportunities that we can offer to young people are offered on a fair basis."
He said he would be writing to another 50 companies asking them to join the scheme.
Intern Aware, the campaign for fair internships, said the government's social mobility rules did not go far enough.
Ben Lyons, co-director of Intern Aware, said: "This scheme will help young people without parents in that cosy dinner party circuit where a quiet word can lead to a job. Smart employers know that it makes business sense to hire on merit, not background.
"However, not enough is being done to ensure that interns can afford to take up opportunities. If Nick Clegg is truly committed to social mobility he will make sure that HMRC enforces the minimum wage for interns."