The total has fallen from 110,000 in 1988 to 74,500 in January this year - a drop of more than 30 per cent, according to the annual census of the Independent Schools Information Service, which covers about 80 per cent of pupils in independent schools. But the decline in boarding has been only 2.4 per cent in the past two years.
Day pupil numbers, meanwhile, have risen from 344,000 to more than 400, 000 over the same period.
A recent phenomenon has been the rapid growth in occasional (or "sleepover" boarding), introduced in response to parental demand by schools that were formerly fully boarding. About 13,000 pupils spent an average of eight nights each in school during 1997, up 11 per cent on the previous year.
The proportion of boarders varies according to the age and sex of pupils.
Nearly one in four pupils taught by members of the Headmasters' and Headmistresses' Conference, which represents the major public schools, is a boarder, compared with only one in eight at schools belonging to the Girls' Schools' Association and one in 10 at prep schools.
The Boarding Education Alliance, set up a year ago at the instigation of Edward Gould, headmaster of Marlborough College, has won the support of more than 180 schools for a three-year, Pounds 250,000 campaign to counter the decline in boarding.
The number of pupils boarding in the maintained sector has also fallen - from 4,400 in 1992 to 3,700 last year - but, there too, the rate of decline has slowed recently.