The average fee for attending a top private school has risen past £17,000 a year for the first time, despite climbing at the slowest rate for more than 20 years, according to the annual census of leading independent schools.

Data collected from the 1,300 members of the Independent Schools Council (ISC) – an umbrella body that ranges from famous public schools such as Eton and Harrow to hundreds of smaller prep schools – showed that a child attending a private secondary school as a day pupil would cost around £15,000 a year, while a boarder would pay £33,000.

The figures showed an increase of 3.3% compared with 2017, which the ISC said was the slowest annual rate of increase since 1994. However, independent school fees have outstripped the rate of inflation over that period.

The average annual fee for a day pupil was around £13,000 three years ago but 10 years ago the same education cost £9,600 a year. Boarding fees have risen by 50% in the last 10 years, having cost on average £22,000 in 2008.

In comparison, state schools in England receive around £4,000 to £6,000 per pupil depending on location and additional allowances for language and disadvantage, as well as access to funds for buildings and maintenance.

Julie Robinson, the ISC’s general secretary, said: “It is important to remember that these families save the government money by not taking up state school places. ISC schools save the taxpayer £3bn a year from students not being in state education.”

Despite the increases in fees the appetite for a private education seems stable, with a rise in students attending the council’s member schools to 529,000, the highest number since its records began in 1974. Much of the increase was accounted for by new schools joining the group.

The census showed another strong increase in the number of pupils coming from China and Hong Kong, which make up the largest group of non-British nationals in independent schools. Together they account for 14,000 of the 53,000 non-British pupils, including those with parents living inside and outside the UK.

Pupils from Russia account for 2,800 of the total, while there are just under 4,000 from the US.

Ethnicity data showed that ISC schools in England had a slightly higher proportion of pupils from non-white British ethnic groups, compared to state schools in England.

The ISC said that its member schools were offering increasing amounts of fee assistance worth nearly £1bn a year to families of pupils, to help them meet the heavy costs involved. The census found that 171,000 pupils were receiving help of some sort with their fees.

“In recent years independent schools have raised money to subsidise the fees of lower income families, widening access. Students from disadvantaged backgrounds, who are on bursaries at ISC schools, often go on to secure further financial help in order to attend top UK universities,” said Barnaby Lenon, the ISC chair and a former head of Harrow school.

“This census tells us that a third of pupils at our schools benefit from reduced fees. Independent schools are committed to educating the broadest spectrum across society.”

In recent years independent schools have come under attack by both Labour and Conservative MPs for hindering social mobility, although the government has dropped plans to force private schools to support state schools or risk losing their charitable status.

The census notes that only three-quarters of its member schools now operate as charities, with an increasing proportion either dropping or foregoing charitable status.