Public school class sizes rose last year because of an exodus of 3,000 teachers.

The typical class size in the independent sector, where fees can reach £20,000 a year, increased from 9.7 in 2001 to 10.1 pupils per teacher, according to the Department for Education and Skills (DfES).

Small classes are a major reason why parents choose to go public and the Independent Schools Council (ISC) said it was querying the figures as its own census showed average sizes fell.

The ISC said its 2002 average class size was 10.47 pupils per teacher, down from 10.55 the year before. It also said there were more than 800 extra teachers working in the private sector last year, not 3,000 fewer.

Average class sizes in prep and public schools have been on a downward trend since the Sixties and were still well below those in English state schools last year.