A quarter of secondary school students have sought help from private tutors, a survey has revealed.
The Sutton Trust charity, which commissioned the research, said the latest findings have fuelled their concerns about unequal opportunities for lower income families.
More than one in four (27 per cent) young people in England and Wales said they have used private tuition, compared to 18 per cent in 2005, according to a survey of more than 2,300 children aged between 11 and 16.
Pupils in London were much more likely to have received private tuition than the rest of the country, with two in five pupils from London (41 per cent) having had tuition at some point.
Almost half (47 per cent) of those who had used private tutors said the main reason was to help with their school work in general, according to the survey.
But one in three students said they had received extra private tuition to help with their GCSE exams – and 27 per cent sought help from a private tutor for a school entrance exam, the poll found.
Sir Peter Lampl, chairman of the Sutton Trust, said: “The fact that it (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.
”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.“
The social mobility charity has said that the government should introduce a voucher system – funded by the pupil premium – to ensure lower-income families could purchase extra educational support.
Meanwhile, the National Education Union (NEU) has called for all private tutors to face the same criminal record checks as classroom teachers before being allowed to work with children.
Ian March, from the NEU, told BBC News: ”It's important that tuition is not an unregulated industry.
“It's absolutely vital that the government makes sure everyone who works as a tutor has a DBS check as people may work as a private tutor when they've been barred from working in other teaching environments.”
Nadhim Zahawi, minister for children and families, said: “Nothing is more important than the safety of children and if parents decide to employ a private tutor for their children they should assure themselves of a tutor's suitability beforehand.
”There are a range of checks parents can carry out, including checking the outcomes of the teacher misconduct panel hearings to see if a private tutor has been prohibited from teaching.“
Earlier this year, academics from the University College London’s Institute of Education (IoE) suggested that families who use private tutors to help their children pass the grammar school entrance test should pay extra tax to allow poorer children to have the same support.