Wealthy parents are being lured away from the private sector as state schools enjoy exam success and improved pupil behaviour, according to the Good Schools Guide.
Over the past decade there has been a surge in the number of state schools featured in its editions, which have traditionally listed private schools where fees can top £30,000-a-year.
The improving state school standards mean increasing numbers of parents who have the money to educate their children privately are opting for state-run schools instead.
Lord Lucas, editor-in-chief of the widely respected guide, said better behaviour and results at state schools were "attracting families who can afford private".
The assessment was welcomed by the Government, which claimed over a million more pupils are being taught in state schools rated "good or outstanding" now compared to 2010.
The news also received some praise from the Headmasters' and Headmistresses' Conference (HMC), which represents nearly 300 independent schools.
In an interview with the Times Literary Supplement Lord Lucas said the "rise of the state system" was a "very difficult thing for the independent sector, as a whole, to resist".
When the Good Schools Guide was first published in 1986 it featured just 10 state schools, whereas last year 265 of the 888 featured schools were from the state sector, according to reports.
The figure has more than doubled since 2005, when the guide recommended 130 state schools.
The HMC said it was "good to see education for all pupils improving" and attributed some of the success to partnerships with independent schools.
However, responding to suggestions the private sector was under threat, it stressed "everyone can win" and said the education sector was "not like the football league".
Christopher King, chairman of the HMC, said it was an "ongoing challenge" to provide future-proof education for children "at a fee parents can afford".
In May last year the conference said the number of children being educated in the independent sector was at a record level.
A Department for Education spokesman said: "Our reforms have been underpinned by a commitment to social justice and fairness – that means achieving educational excellence for everyone, everywhere, regardless of their background.
"Thanks to our reforms 1.4 million more pupils and counting are being taught in good or outstanding schools than in 2010, so it's no wonder more parents are choosing for their children to be educated in the state sector."