Pupils given private tuition achieve on average just half a grade higher at GCSE than those who have had no extra tuition, research from London university's institute of education shows.
Academics found boys increased their performance by three-quarters of a grade, often sufficient to push them up a whole grade.
But maths tuition had little effect on girls' performance. There was no evidence that out-of-school coaching in English had any effect on GCSE grades for either boys or girls.
Twenty-seven per cent of the 300 Year 11 pupils surveyed by academics admitted using a private tutor at some point during their schooling, usually in maths or English.
Gender disparity in the effectiveness of the tuition was explained by a lack of confidence among girls, whose grades often need little improvement in reality. A general increase in the performance of girls between Years 9 and 11 also makes it more difficult for tutors to generate extra improvement.
Judy Ireson, professor of psychology in education and author of the report, said research shows coaching offered by schools raised grades.
"Most people assume tutoring is the answer to boosting children's attainment," she said. "But there is no regulation of private tutors, so there is no guarantee that the tutor you get is going to be good."
The Prime Minister, however, is among the many parents who believe that private tuition, which costs an average pound;20 an hour, reaps benefits.
Tony Blair hired teachers from Westminster school, one of the highest-achieving private schools in the country, to tutor his eldest children.
Parent Rashmikant Shah, of Portsmouth, Hampshire, has employed an English tutor for his 15-year-old daughter, Anjlee, since primary school. Last year, she sat English literature and language GCSEs, achieving As in both.
"The education level at her school is very high, but private tuition backs her up," Mr Shah said. "It gives her a head start, and puts her above the other students."
Robert Breslin, maths teacher at Holloway school, in north London, thinks tuition can reinforce classroom teaching and said: "If you go over something again and again, it's more likely to stick. It does not matter if the tutor teaches a different method: if you have more tools, it is only going to help you."
Clive West, director of Anysubject private-tuition agency, said: "Tutors are often seen as the whipping boy. But there can be good and bad teachers in schools.
"Parents are not dimwitted. They do not come back to us day in and day out without reason. They can see progress for their money."