Hundreds of schools across the south of England and Wales are expected to close or vary their hours this week, in an effort to combat the extreme heat and conditions labelled as dangerous by education unions.

School leaders said they had warned parents about potential closures on Wednesday and Thursday, with many opting to end the day early to avoid the worst of the afternoon heat.

Headteachers say they are deploying fans and portable air conditioning in classrooms where possible, although several reported that supplies of fans at local hardware stores had already sold out.

Daniel Kebede, general secretary of the National Education Union, said it could be dangerous for schools to remain open unless they were able to mitigate the heat, and warned: “Our Victorian school buildings have become greenhouses.

“The government must step up. We need urgent, massive capital investment to retrofit our ageing school estates with proper ventilation, shading, and climate-resilient cooling infrastructure.

“Expecting schools to carry on like normal right now is dangerous.”

With temperatures expected to reach 40C within a “red zone” affecting London, much of the south-west of England up to Birmingham and parts of Wales, schools without air conditioning or adequate ventilation will struggle with the heat disrupting activities as well as their ability to support children with medical conditions or special needs affected by high temperatures.

London boroughs are among the worst affected, with many primary schools allowing parents to collect their children early. Several schools in Berkshire and Wiltshire are to close, with St John’s Marlborough secondary school in Wiltshire among those telling parents it will close from lunchtime on Tuesday and remain closed on Wednesday and Thursday “due to red alert for heat”.

One parent in Bristol who contacted the Guardian said her children’s primary school was closing after lunch on Tuesday until Friday, in contrast to the 2022 heatwave when the city’s schools remained open.

“I am worried that this total lack of planning for the heat bodes poorly for kids and working families as summers heat up. School closures disproportionately impact kids and families where both parents work, or single parents, and particularly those in more precarious employment who may lose several days’ income,” said Lucy, who only gave their first name.

The Department for Education’s policy in England is “normally” for schools to stay open during hot weather. The DfE said: “School attendance is the best way for pupils to learn and reach their potential, and hot weather can usually be managed safely.

“School leaders should make sure they take any steps necessary to make sure children are safe and comfortable, and the heat-health alerts can guide this.”

However Stacey Booth, a national officer for the GMB union representing 100,000 school support staff, has called on the DfE to act with “high urgency”.

“Union members are increasingly contacting GMB in desperation, as current measures to cool their classrooms, kitchens and workplaces are inadequate in the current heatwave,” Booth said in a letter to the DfE’s official overseeing school estates.

Booth said the DfE should be funding schools to buy more fans and cooling units as well as extra supplies of water if needed.

Paul Whiteman, general secretary for the National Association of Head Teachers, said: “Many school buildings are ill-equipped to deal with extremes of temperature –especially smaller primary schools, often housed in older buildings.”

Whiteman said the government “really does need to act more urgently to improve and modernise school buildings, including a focus on ventilation and potentially air conditioning”.

Bridget Phillipson, the education secretary for England, told parliament on Monday: “We do recognise the need to make sure that as we refurbish and rebuild schools across our country, we make sure that they are well placed to cope with some of the fluctuations in temperature that we’re seeing.”

In the meantime, however, Phillipson said it was “for school leaders as to how they manage that best”.

In Wales the government has written to council education directors, highlighting its advice about the safety of children and staff in hot weather, particularly children under the age of four as well as those who are overweight, on medication or with disabilities or complex health needs.

Schools in the worst affected areas have warned parents that transport arrangements such as school buses could be disrupted by the hot weather, and most have loosened uniform requirements, in many cases allowing pupils to wear PE kit instead.