Three in 10 youngsters have been given extra private lessons outside of school, research suggests.
Children from wealthier families are twice as likely to have received additional help than their poorer classmates, while those from minority ethnic backgrounds are more likely to have had a tutor than white pupils.
The Sutton Trust, which commissioned the Extra Time report, said private tuition is the "hidden secret" of British education, and that in an "educational arms race" it reinforces the advantages of youngsters from richer homes.
A poll of more than 2,600 secondary school children, conducted as part of the study, found that 30 per cent of youngsters said they have received private or home tuition at some stage.
This was up from the 25 per cent of 2,500 youngsters who said the same last year, and approaching double the 18 per cent who said they had received extra help in 2005.
More than a third (35 per cent) of those from more advantaged households said they had received private tuition, this year's survey found, compared with less than a fifth (18 per cent) of those from less-well-off families.
White children 'less likely to get tuition'
In addition, over half (56 per cent) of Asian pupils and 42 per cent of black students said they had had a tutor, compared with 25 per cent of white children.
The survey also showed that a child's chances of getting extra help varied depending on where they lived.
While just over one in 10 children across England and Wales (11 per cent) said that they had been given private tuition in the past year, this rose to almost one in four (24 per cent) among those living in London.
The most common reason for extra tuition was general help with school work (44 per cent), while 38 per cent said they had been given tutoring for specific GCSEs and 28 per cent said it was for a school entrance exam.
The study also used international data published by the Organisation for Economic Cooperation and Development, based on tests taken by 15-year-olds in over 70 countries to examine how private tuition varied between nations.
It found that, on average, Year 11 pupils in England spent 9.5 hours a week in additional instruction – help outside of normal lessons provided by schools, families or private tutors.
Out of 22 countries examined, teenagers in England spent significantly less time getting extra help than their peers in 12 other countries.
The study also concluded that bright, poor teenagers in England spent around seven hours a week, on average, in extra lessons, while less clever, richer youngsters got around double this – about 15 hours on average.
Poorer pupils also got less help with their homework, with half of the most disadvantaged 15-year-olds saying that their parents regularly helped them with their studies, compared with 68 per cent of their better-off classmates.
In a foreword to the report, Sutton Trust chairman Sir Peter Lampl said: "Private tuition is the hidden secret of British education. Within an educational 'arms race' that entrenches advantage for those who can afford private school fees or homes close to good comprehensives and grammars, it has remained largely in the shadows."
He added that there are "important social mobility issues" surrounding the differences in children who get extra tuition.
Report author Dr John Jerrim, of the UCL Institute of Education, said: "These figures show that in the UK children from poorer homes receive significantly less help with their studies outside of school than in many of the other countries surveyed.
"As a result, children of high ability from low-income families are not receiving the kinds of educational opportunities they should."
The trust said it was calling for schools to establish "homework clubs" to give poorer pupils the support they need, and urged government to introduce a voucher system, using money set aside for disadvantaged children, to provide addition tuition for these youngsters.
The Ipsos MORI poll questioned 2,612 11-16-year-olds in England and Wales between February and May.