The higher test results achieved by pupils in England's private schools are being used by ministers trumpeting the improved standards in the state system.

The effect is to inflate the results by one percentage point.

The government sets targets for the results it wants 11 year olds to achieve by the time they finish primary school - new ones have just been announced for 2004.

Prep schools do not have to follow the national curriculum and so are not obliged to enter their pupils for the national curriculum tests.

In practice, probably about two thirds do so - some 500 schools in all. There are more than 18,000 state primary schools.

Invisible targets

When the achievements of pupils are quoted by the Department for Education, it includes prep schools' results - even though a separate state-only figure is available in the same set of data.

A spokesperson for the department said: "It is right that we include all pupils both in the targets and in the results we report."

She added: "Although independent schools are not required to set individual school targets this does not prevent their inclusion in our national targets."

But it is not clear how independent schools are included in the targets.

The department has published a list of the targets in English and maths for individual local education authorities, which are responsible for the maintained schools.

The average of those lists are the national targets. There are no targets for independent schools.