Family holidays are becoming the “preserve of the middle classes” because of new penalties for parents that take children out of school in term time, according to the NUT.
The union’s general secretary Christine Blower has warned that poorer families risk missing out on holidays because they cannot afford the high cost of travelling outside of term-time.
Under new rules introduced by former education secretary Michael Gove in September 2013, heads are now only able to grant leave in "exceptional circumstances" and parents can be fined £60 per pupil for taking term-time holidays without permission.
Ms Blower said holiday companies “jack up the cost of holidays” outside of term-time.
“The upshot is, if you have a reasonable amount of disposable income it might not be what you want to do but it's something you can afford to do,” she said.
“But if you are a family who doesn't really have much disposable income – and we know that the poor are getting poorer – then they are going to miss out on this.
“Education in school, with qualified teachers, is central…but a lot of things [children] can do outside the classroom also contribute to their development.”
She said parents should not take children out of school “at the drop of a hat” but that a family holiday could “be a very positive thing in a child’s life”.
Last year, Essex County Council took the parents of three children to court after the couple refused to pay the £120 fine imposed on them when they took their children out of school during term time.
It was the first case of its kind, and the father, City banker James Haymore sought to challenge the legal action, claiming the law breached their human right to a family life.
The case was eventually dropped in September after the family moved back to their native US.
A resolution due to be debated by delegates at the NUT's annual conference in Harrogate this Easter argues that despite recent advice from heads clarifying their interpretation of the rules, the regulations still unfairly impact on working parents, especially the low paid.
It says: “Conference understands that taking children on holiday is not the same as persistent truancy.
“Holidays can provide valuable experiences and outdoor learning opportunities. Giving families time to be on holiday together will also have social and emotional benefits which can be of lasting value and support to schoolchildren.”
The resolution also says that much more pressure needs to be put on travel companies by the government to change pricing structures that see fares soar during school holidays.
The Department for Education said taking children out of school without permission for a holiday is a criminal offence.
A spokesperson added: "The myth that pulling a child out of education for holidays is harmless to their education has been disproved by evidence. Allowing pupils to regularly miss school can be hugely detrimental to a child's life chances.
"Heads and teachers are now firmly back in charge of their classrooms thanks to our plan for education, and new flexibility over term dates allow them to set term breaks outside of peak times."