"The government's pledge to improve class sizes in primary schools must not be fulfilled at the expense of children in secondary schools," says John Dunford, general secretary of the Secondary Heads Association.

Mr Dunford said that extra funding to reduce primary school class sizes had drawn money away from the secondary sector, leaving "secondary school heads and governors with no alternative but to increase class sizes, as they struggle to balance their budgets".

John Dunford wants the government to face its responsibilities over class sizes

The union claims that average class sizes in secondary schools have risen in the past year - from 21.9 to 22 pupils - and that in the past decade there has been a rise in the percentage of pupils being taught in classes of more than 30 - rising from 6.9 in 1989 to 10.9 in 1999.

"Put together with the crisis in teacher recruitment in secondary schools we have an acute situation from which the government cannot escape its responsibility," said Mr Dunford.

"The government is proud of its record in reducing class sizes in primary schools, but on all the measures in its class size statistics, the situation in secondary schools is getting worse."

The warning from the Secondary Heads Association follows the comments of the Education Secretary, David Blunkett, earlier this week, in which he said the government was on target to reach its class size reduction promises for the first three years of primary school.