Professionals from working class backgrounds are paid £6,800 less than colleagues who had affluent upbringings, research has found.
Women and ethnic minorities face a "double disadvantage" that means their earnings are hit even harder, according to the Social Mobility Commission report.
The 17% pay gap shows that Britain remains a "deeply elitist" society, chairman Alan Milburn said.
Medicine, law, journalism and academia continue to be dominated by the middle classes, the study found.
Mr Milburn, a former Labour cabinet minister, said: "This unprecedented research provides powerful new evidence that Britain remains a deeply elitist society.
"Too many people from working class backgrounds not only face barriers getting into the professions, but also barriers to getting on. It cannot be right that they face an annual class pay gap of £6,800.
"Many professional firms are doing excellent work to open their doors to people from all backgrounds, but this research suggests much more needs to be done to ensure that Britain is a place where everyone has an equal chance of success regardless of where they have come from.
"How much you are paid should be determined by your ability not your background. Employers need to take action to end the shocking class earnings penalty."
The research, carried out by academics from the London School of Economics and University College London, looked at the average earnings of people from different backgrounds within the professions and found those who had a poorer start in life lost out by around £6,800 a year.
It found the gap was partly caused by differences in educational background, along with the tendency among the middle classes to work in bigger firms and head to London.
But even when professionals had the same educational attainment, role and experience, those from poorer backgrounds were paid an average of £2,242 less, the study found.
Finance had the biggest earnings gap at £13,713, followed by medicine on £10,218, and IT on £4,736.
The study found men from professional and managerial backgrounds earn 21% more than working class women in the professions.
All black and minority ethnic professionals, except those who are Chinese, earned less than similar white colleagues.
The report found professionals from poorer backgrounds may be less likely to ask for pay rises, and do not have the same networks and opportunities as their counterparts.
It suggested there could also be conscious or unconscious discrimination among employers.
The study used information from the UK Labour Force Survey, which has more than 90,000 respondents.
It found social mobility is "the norm and not the exception" because 48% of people rise up the ladder from their parents' position compared with 31% who slide down.
But there continue to be "strong barriers" to equality of opportunity.
Sam Friedman, from the LSE, said: "We have found evidence of a powerful and largely unacknowledged pay gap within the professions. There are a number of reasons for this such as higher educational attainment among the privileged. But even when these factors are taken into account, this gap remains significant."
TUC general secretary Frances O'Grady said: "Far too many people are missing out on the pay and opportunities they deserve, simply because of their background.
"We need to get more working class people into better paid jobs.
"A good start would be ensuring that workers have seats on company boards, bringing a reality check to corporate Britain.
"TUC research has shown that countries with workers on boards have higher employment rates, lower poverty rates, and invest more in research and development."
Liberal Democrat leader Tim Farron said: "To get your foot in the door, too often it's not what you know but who you know.
"I have never felt as common as when I entered the House of Commons.
"It's time for the Government to not just say the right thing, but do the right thing, by investing in education and ensuring every person is given a chance to succeed."
A Government spokesman said they were working with schools, colleges and businesses to overcome barriers to social mobility.
"We are looking at ways to deliver more good school places in more parts of the country, investing in improving careers education, transforming the quality of further and technical education and opening up access to our world-class higher education system," the spokesman said.
The UK's 10 best-paid jobs
The UK's 10 best-paid jobs
Flying high literally as well as metaphorically, pilots top the salary scale this year. There are various routes into the job, from university courses to airlines' own training schemes; private training will set you back as much as £50,000 to £60,000.
"I most enjoy the variety of the job, working as a team, to a schedule, with the responsibility of safely transporting hundreds of people to their destination. I would advise aspiring pilots to thoroughly research training routes, understand the serious financial outlay required with no guarantee of a job after training and to consider other routes such as flight instruction."
Not all CEOs are making millions - but they're doing pretty nicely on average. There's no clear route into the job; an MBA can help, but many bosses have worked their way up from the bottom and many simply started their own firm.
"There is an old saying, it's very lonely at the top, which is usually the case for a CEO. You're the leader and you're responsible for everything, meaning you take the credit for the successes and feel the most pain for the companies' failures."
Most air traffic controllers train through National Air Traffic Services (NATS), which has programmes for college leavers who have studied at BTEC ND/HNC/HND level, university students on a sandwich year and graduates. NATS says the most important attribute for acceptance on a course is natural aptitude.
"Much of the time, the job is routine and capable of being done by many people. What you are paid for, however, is your ability to make the right choices and decisions under extreme pressure."
The high pay reflects both the heavy workload and the length of training required - a five-year degree course in medicine, a two-year foundation programme of general training and then another three years specialist training.
"You will get paid well and (despite recent changes!) you will get a very good pension. You will, however, work hard for your money with less recognition than previous years."
This is a job where most people work their way up, although qualifications like the Chartered Institute of Marketing's Professional Diploma in Marketing or the Institute of Direct and Digital Marketing's Diploma in Direct and Interactive Marketing can help. You'll probably need three or four years' experience before you can move into management.
"It's great working in such a varied job - there's something new every day. I enjoy the travel, too. But it's fast-paced, very hard work, and there's a lot of technical data analysis work involved."
It's not surprising that IT and telecoms directors earn good money. The job requires both business and technical skills, and the stakes are high: smoothly-running IT systems are crucial for most businesses and the cost of getting things wrong is high.
"It's a high-pressure job, and the hours can be unbelievably long, but it's extremely rewarding when a project goes well. The biggest problems are staying on top of changes in technology and being clear about what's actually achievable within a budget."
Financial managers and directors need to be qualified accountants and, particularly in larger organisations, have strong general business skills. And the job carries huge levels of responsibility, from keeping the company in good health financially to making sure it complies with the law.
"What I enjoy about the job is the way it's central to the business, the way I'm involved in creating long-term business plans. Compliance [with regulations] can be a bit of a pain; it takes a lot of hard work."
Until recently, the only way to become a senior officer was to work your way up through the ranks. Each police force sets its own recruitment process and selection policy, though there's a basic national framework for evaluating competency. This year, though, the College of Policing has started appointing superintendents from outside the force - so you might be in with a chance.
"I love the opportunity to take on a vast array of roles which would never be possible in a civilian organisation and to progress through a clearly defined promotion structure, should I prove to be good enough. The training I have received has been quite brilliant, enhancing my life skills as well as my ability to do my job well."
If your vision of a bank manager is of a middle-aged man in a panelled office dictating stern letters to his secretary, think again. It's now about managing a team and, especially, attracting new customers and monitoring branch performance. There are two ways to get to be one: either through a bank's graduate scheme, or by starting off in a customer service role and working your way up.
"For me, what's great about working in a branch is that anyone could be a branch manager, you don't have to have a certain skill set. Having a background in customer service, whether it's in a bank or with another retailer, certainly helps. Some other skills that would help are organisation, patience, tolerance and empathy. After 27 years I can genuinely say that that not only do I love coming to work every day, I have also made lifelong friends here. For me, it's not just a nine-to-five job."
Once you're qualified as a teacher, it's a question of working your way up by gaining extra responsibilities - by becoming a head of department, a head of year, or a specialist in something like special educational needs. At head teacher level, the job is far less about teaching itself, and far more about managing staff and budgets and creating a productive, disciplined learning environment.
"It's a common complaint that teaching is all about performance targets these days, and there is some truth in that. It's a question of balance, though. The most rewarding part of the job is knowing that you're bringing the best out of every single child by creating an environment where they feel safe and motivated to learn."