Pupils at private schools are three times more likely to score A grades or better at GCSE subjects than students at state schools, a report suggests.
Nearly six out of 10 GCSE exams taken in private schools received grade A or A*, the Independent Schools Council (ISC) report says.
That compared with only two out of 10 nationally, the ISC added.
A spokesman for the Department for Children, Schools and Families said standards were higher across the board.
The top school based on the average points score per pupil was Harrow, where boys got the equivalent of an average of more than 11 A*s each this year.
The report found 26.8% of entries in independent schools was awarded an A* grade, up from 26.5% last year. The national average was 6.4% this year.
'Good job'
The figures came after a row last week over claims by exam boards that standards in private schools were falling.
The AQA board had said standards were rising in comprehensive schools while those in private schools were on the wane.
ISC chief executive Jonathan Shephard said the figures released on Saturday showed the board's claims last week did not reflect the true picture.
"The reality is our schools do a good job," he said.
The ISC also said last week that many of its schools were being penalised because they were teaching the International GCSE (IGSCE), which was not included in figures, and is regarded as a more challenging exam.
"Plainly it is important to include IGCSE results in the overall calculations," Mr Shephard said.
"This will be done, we have no doubt, in the near future. Meanwhile, the results put out by the exam boards which excluded the IGCSE showed only part of the picture."
A spokesman for the Department for Children, Schools and Families, said: "It's not groundbreaking news that children at independent schools, many of which are selective, show better results.
"We're raising standards across the board and A* to C results in the maintained sector have gone up steeply compared with independent schools. The IGCSE issue is a red herring," he said.
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