More are forced into streamed classes

Staff who oppose selection are also thought to be behind the trend.

Across all secondary subjects, just 43 per cent of lessons are taught in sets, according to the latest inspection figures.

But the decline appears to be steepest in maths and science - the subjects where ability grouping is considered most crucial.

With as many as half of physics teachers lacking degrees in the subject, a lack of qualified specialists is feared to be fuelling the trend.

Critics warned that schools could be putting under-qualified teachers in front of mixed groups rather than the brightest pupils to disguise their poor grasp of the subject matter.

The news came as a report from the Campaign for Science and Engineering claimed ministers were counting business studies and textiles teachers when they claimed there had been an increase in the numbers training to teach the sciences.

The increase in mixed ability teaching, which was uncovered in a Commons written answer, is highly embarrassing for Labour. The party came to power in 1997 pledging a return to setting.

Tony Blair formally abandoned Labour's commitment to mixed ability teaching in the party's 1997 manifesto.

It was seen as an acknowledgement that rigid mixed-ability teaching - a totem of the progressive education lobby - had failed a generation of youngsters.

But figures from Ofsted show a drop of 15 per cent in the number of maths lessons taught in sets - from 88 per cent in 2002/03 to 75 per cent in 2005/06.

In science, 63 per cent of classes were setted last year, against 74 per cent in 2002/03 - again a drop of 15 per cent.

The Tories, who uncovered the figures, claimed too many teachers were clinging to outdated theories which hold that children-suffer from being divided into sets.

Tory schools spokesman Nick Gibb said: "It is alarming that ten years into a Labour government we are still in a position where the overwhelming majority of lessons still take place in mixed ability classes.

"I am convinced this is a key factor in low standards in comprehensive schools.

"If we want to raise standards, we must tackle this problem."

He added: "This is ideologically driven - there is a strand of opinion among teachers that the bright ones can help the less able ones.

"But I believe the bright pupils become bored and the less able get frustrated.

"The evidence is very clear - you get huge increases in educational attainment if you set children by ability."

Mr Gibb said that under a Conservative government, under-performing schools would face black marks in Ofsted inspections if they failed to put pupils in sets for all subjects.

The figures are based on Ofsted visits to secondary schools over four years beginning in 2002/03.

Inspectors recorded the number of lessons taught in sets. Of 55,271 lessons observed in 2002/03, in all subjects, 44 per cent were grouped by ability.

In 2005/06 inspectors saw fewer lessons after introducing new "shorter, sharper" visits.

Of 19,692 classes seen, 43 per cent were grouped by ability.

While the prevalence of ability grouping had dipped slightly in English - from 57 per cent to 54 per cent - there were sharper drops in maths and science.

A Department for Education and Skills spokesman claimed last night that comparisons between data from different years were unreliable.

He said: "Ofsted do not inspect the same schools every year.

"We have encouraged schools to use setting since 1997, and will continue to do so."