The county council told schools they were expected to remain open during the 40C heatwave and child absences should be treated as “unauthorised”.

Herts County Council has come under fire after a leaked email, sent by a headteacher to parents, quoted it giving the instruction ahead of blistering heat this week.

It reads: “Further to my previous email and requests I have subsequently received about closing early on Monday and Tuesday, email communication from Hertfordshire County Council to headteachers this morning has stated the following:

“The expectation is that schools remain open and therefore parents should be sending their children to school.

“Absences due to the heatwave, unless unavoidable, should be treated as unauthorised.”

One parent, who wished to remain unnamed, branded the move “unduly harsh” particularly for those with disabled children and said it could result in parents being fined.

On Tuesday, another mum claimed a teacher at her child’s school in the Watford area described Monday as “unbearable”.

“The children were kept indoors all day, they were all hot and miserable, there was no air breeze at all, they had no relief from the heat,” she added.

“I spoke with some parents from my child’s class and they said their child was sent home due to being sick, another child was sent home due to heat exhaustion.”

She said some parents planned to send their kids in as instructed before changing their mind on Monday evening after seeing the effects.

The mum added: “Why is it right that job centres with air conditioning have closed but we keep our children in school to overheat?”

Concerns were also raised over teachers, kitchen staff and caretakers being forced to work in sweltering conditions.

A HCC spokesperson said: “The advice we passed on to Hertfordshire schools is the Department for Education’s advice on how to manage in a heatwave, and we are encouraging schools to follow this national guidance.

“Ultimately, headteachers know their schools and school communities best, and are responsible for managing their own local circumstances, including using their discretion to authorise absences where there are exceptional circumstances.”

A petition demanding schools close during the heatwave attracted over a thousand signatures but it is thought most remained open.

One that did shut was Watford Grammar School for Girls, which moved teaching online for both days citing the “dangerous” temperatures.