Private tutoring is the "hidden secret" allowing better-off families to stop their less able children from failing at school, research suggests.
Bright teenagers from poor homes get half as much extra tuition as less able pupils from wealthier homes, a report for the Sutton Trust charity suggests.
Disadvantaged pupils also miss out on homework help from parents, with half of poor 15-year-olds getting support.
This compares with two-thirds of pupils from the most advantaged homes.
On average, GCSE-year pupils in England spend 9.5 hours per week receiving extra tuition, help with homework from parents or support from school.
The Sutton Trust said private tuition was the "hidden secret" of British education in an "educational arms race" that reinforces the advantages of youngsters from richer homes.
Tutoring is a £2bn a year industry, according to the trust, but a lack of transparency means that it is difficult to get an accurate picture of the market.
The trust claimed to be "lifting the veil on its prevalence" to shine a light on "important social mobility issues" surrounding the differences in children who get extra tuition.
'Education arms race'
The study found, as might be expected, that pupils from different income groups receive different levels of extra support.
Those with with well-off parents are receiving 2.5 hours more of extra tuition a week than less well-off pupils, the report said.
All in all more than a third (35%) of those from more advantaged households said they have received private tuition.
This compared with less than a fifth (18%) of those from less-well off families.
But the difference was most stark when comparing high achieving GCSE pupils from poor backgrounds and low achieving pupils from wealthy backgrounds.
The study also concluded that bright, poor teenagers in England, spent about seven hours a week on average in extra lessons, while less clever, richer youngsters get more than double this - about 15 hours on average.
A poll of more than 2,600 secondary school children, conducted as part of the study, found that 30% of youngsters said they have received private or home tuition at some stage.
The research also revealed a racial dimension with 56% of Asian and 42% of black students saying they had experienced tutoring, compared with 25% of white children.
Poorer pupils also get less help with their homework, with half of the most disadvantaged 15-year-olds saying their parents regularly help them with their studies, compared to 68% of their better-off classmates.
Report author Dr John Jerrim said children of high ability from low-income families were "not receiving the kinds of educational opportunities they should".
The trust is calling for schools to establish "homework clubs" to give poorer pupils the support they need.
It also urges government to introduce a voucher system, using money set side by central government for disadvantaged children.
The Ipsos MORI poll commissioned as part of the research spoke to 2,612 11-16-year-olds in England and Wales between February and May.