Almost one in four young people receive extra tuition paid for by their parents outside of school time, according to research.
It also suggests that the richest families are around four times more likely than the poorest to shell out for extra-curricular activities such as sport, music and drama.
The study raises concerns that pupils from advantaged homes whose parents can afford to pay for extra coaching and after-school clubs are gaining an unfair advantage over their poorer classmates, who are missing out.
Education charity The Sutton Trust, which commissioned the study, said it is calling for a proportion of existing government funding designed to boost the academic achievement of disadvantaged children to be handed to low-income families to help them cover the cost of a wide range of activities.
The report, based on polls of parents and children, as well as a new analysis of existing government figures, found that overall, 23% of the 2,800 11-16-year-olds questioned said they have received private or home tuition.
Those in London are the most likely to get extra help, with more than a third (37%) saying they have had coaching in a subject outside of the classroom.
A separate analysis of data drawn from the Office for National Statistics' Living Costs and Food Survey found that more than a third (35%) of the richest households - those earning over £52,000 a year - had paid for extra-curricular activities including crafts, dancing, music, drama, art, sports and languages among others, for their children in the previous three months.
This is around four times as many as those from the poorest homes - those with incomes of less than £14,000 annually (9%).
The study also found that overall, around three quarters of all parents (76%) across all social groups say their sons and daughters have been involved in some form of regular extra-curricular activity over the last year.
This poll, which questioned 309 parents of five to 16-year-olds, also showed differences between social classes, with participation in after-school clubs around 15 percentage points higher among families with parents in professional or administrative jobs than those with mums and dads working in manual or routine occupations.
More than a fifth (22%) of parents in the higher social groups said they had spent at least £500 on extra-curricular activities for a child in the last year, compared with 10% of those in the lower social groups.
The research report said that all the findings reveal the extent of inequality outside the classroom in the UK.
"By using private tuition both to help with day-to-day schoolwork, and to gain admission to selective schools, richer parents are able to give their children a large academic boost," it concludes.
"This will likely make a big difference to their access to the most selective universities, and subsequently to the highest paying careers.
"Previous research has also shown a positive effect of extra-curricular experience on both education and career outcomes - giving children from richer families another edge."
The Sutton Trust suggested that a proportion of the pupil premium - extra cash handed to schools for the poorest students - should be diverted to low-income families to allow them to widen the range of after-school activities available to them.
Conor Ryan, the Trust's director of research said: "Inequalities in education don't stop after the school bell has sounded. They extend to the range of private tuition and extra-curricular activities available to children whose parents can afford to pay for them. While many schools offer a range of sporting and other activities outside regular school hours, there is still a substantial advantage available to those who can afford it.
"If we are serious about improving social mobility we must narrow the gap in educational opportunities outside of school as well as within the classroom. Offering low-income families vouchers to spend on extra-curricular activities or private tuition would be a step towards this."
A Department for Education spokesman said: "Our reforms to the curriculum and qualifications are closing the gap between the rich and poor and allowing thousands more children to go to good state schools.
"By April 2015 we will have spent £6.25 billion to support disadvantaged children through the pupil premium. Many schools will already be using this to pay for extra tuition and after-school activities to improve disadvantaged pupils' academic progress. It is right that teachers decide how to spend the money to best meet the needs of their pupils."