The National Union of Teachers has hit out at one council's plans to cut school summer holidays, branding it a "recipe for chaos".
Barnsley Council in South Yorkshire will become the first council in England to cut the summer holidays, and will bring in the changes from the 2017/18 academic year.
Pupils and teachers will have four weeks and four days off in the summer - nine fewer than the previous year.
They will enjoy an extra week off in October instead.
Union bosses have branded the move "ill-thought out", and said that claims that it would help children's education by avoiding "learning loss" are "dubious".
Ian Stevenson, regional secretary for the National Union of Teachers, said: "I think any changes to the school holiday pattern has to be for the benefit of children and their education and I don't see how these changes do that.
"We don't accept this notion of learning loss, or that it's significant or that it has any educational merit."
He said the proposed eight-week summer term would be difficult for younger children to cope with, and said that parents and teachers at schools not under the new system would struggle.
He added: "It doesn't make sense to change one local authority's holiday pattern. It's a recipe for chaos."
Headteachers also have reacted angrily to the move, claiming they have not been consulted.
Horizon community college headteacher Nick Bowen said the overhaul would have a severe impact on schools' ability to recruit high-quality staff.
He told The Guardian teachers want and need a chance to "recharge their batteries" as he urged the council to reconsider.
The council said the dates had economic benefits for working parents and had been received positively.
"The decision to alter the borough's term times and holiday dates is based on sound research evidence that shows the maximum length of the summer break should be no more than five weeks for the best educational outcomes," Cllr Tim Cheetham told the newspaper.