One in four secondary school pupils has supplemented school lessons with private tuition, a poll suggests.
The Sutton Trust, which commissioned the survey, said the findings have added to its concerns about educational inequalities, and called on the government to support lower income families.
The poll, conducted by Ipsos MORI, involved 2,381 11- to 16-year-olds in England and Wales, with 27 per cent saying they had used private tutors. In London, this figure stood at 41 per cent.
Sir Peter Lampl, chairman of the Sutton Trust, said it was important to make sure that the "academic playing field is levelled outside of the school gate".
He said: "The fact that [private tuition] is predominantly used to help children do well in a specific test or exam means that those who can afford it are able to give their children a significant advantage over those that cannot.
The state 'should fund private tuition'
"If we are serious about social mobility, we need to make sure that the academic playing field is levelled outside of the school gate by the state providing funding for private tuition on a means-tested basis."
In the survey, 47 per cent of those who had used private tutors said the main reason was to help with their work in general, and 33 per cent said that needing assistance to pass a particular GCSE exam was the reason for having a private tutor.
The foundation said the government should introduce a voucher system - funded by the pupil premium - to ensure that lower-income families could purchase extra educational support.
Private tuition agencies should also provide a certain proportion of their work to disadvantaged pupils for free, the Sutton Trust added.