Teachers are being forced to teach up to five subjects at secondary schools due to budget pressures and are being “exposed” by students for a lack of subject knowledge, an Essex union chief has warned.

Jerry Glazier, general secretary of the National Education Union – Essex Division NUT section, said growing numbers of headteachers believe teachers can lead different subjects, such as geography teachers teaching maths.

The warning comes amid fresh analysis of government data by the School Cuts Coalition which found there were 192 fewer secondary school teachers in Essex in 2016/17 than 2014/15, despite pupil numbers increasing by over 700 to 77,378.

It comes amid ongoing concerns over strained school budgets but the government insists funding remains at record levels and have told heads to tighten their belts.

Mr Glazier said: “There is a massive teacher shortage and it is not being strategically addressed as school populations increase. In parts of Essex it is impossible to find a properly qualified mathematician to fill a vacancy.

“Too often in the secondary sector, we are seeing teachers being asked to teach not one, but two, three, four, or sometimes even five different subjects. They are not replacing teachers because they can’t afford to. It is any combination you can think of.

“Some teachers are willing but too often teachers are complaining to me that they are being told they are a qualified teacher, therefore they can teach any subject, which at secondary school is of course nonsense. If you start teaching subjects which you have no experience of teaching or haven’t got the subject knowledge, you are soon exposed by the students and your ability to be an effective teacher will be diminished, and that means children’s education opportunities are at risk.

“They are vulnerable when they are asked questions around the back of a subject. The kids are very clever. They can spot this. Geography teachers will be asked to teach maths. It’s wrong. Many secondary heads won’t do that but increasing numbers don’t think there’s anything wrong with it.”

Essex will see a £31.2million increase from the national funding formula in 2018/19, according to Essex County Council.

The authority’s cabinet member for education, Ray Gooding, says schools have suffered real term budget cuts, but added: “However, with more Essex schools than ever currently rated as good or outstanding by Ofsted (around 94%), we are confident they are making very effective use of the budgets available to them.

“We hope the new funding formula will ultimately create a fairer and more transparent system, and help ensure children from all backgrounds get the best possible education.”

A Department for Education spokesman said: “The School Cuts figures are fundamentally misleading. We are investing an additional £1.3 billion in school funding, over and above existing plans, through the funding formula, schools in Essex will see an increase of £37.8 million.

“Thanks to our reforms, and the hard work of teachers, there are 1.9 million more children in good or outstanding schools than in 2010.

“There are also a record number of teachers in our schools – 15,500 more than in 2010, with retention rates remaining stable for the past 20 years.”