Thousands of teachers could see major changes to their working hours now that heads and governors have been given powers to transform the structure of the school day.
Legislation that prevented frequent alterations to the length of the day - and ensured that staff and parents had to be consulted before they came into force - has been revoked by the Government.
Unions have warned the decision could have "significant" implications for teachers and parents if school leaders decide to take advantage of the looser regulations.
Schools will no longer have to give six weeks' notice before they change the start and finish times of the school day, and will not have to run a formal written consultation process.
Civil servants have promised teaching unions they will "not be encouraging schools to make particular changes to their day" and any alterations will be a "matter for each school to decide".
But the Department for Education says the rules have been relaxed to give heads more freedom, and the changes follow praise from education secretary Michael Gove for schools that have longer days.
Monkseaton High School in North Tyneside has experimented with start times, allowing teenage pupils to arrive later, which headteacher Paul Kelley believes fits in better with their body clocks.
The regulations were revoked during the summer holidays, so the order could come into force for the start of the new school year. MPs and peers did not need to debate the changes, which were first mooted in last year's white education paper.
Foundation and voluntary-aided schools and academies have never been subject to these regulations, but community and voluntary controlled schools, which are run by local authorities, were bound by them.
NASUWT general secretary Chris Keates said it was wrong that the changes had been made "with no consultation whatsoever".
"Even worse, it is doing this at a time when schools are closed for the summer," she added.
"The DfE may wish to pass off this change in regulations as simply bringing schools into line with others, but the fact is that it will now affect all schools, not just the minority.
"The changes potentially have significant conditions-of-service implications for staff and (could bring) huge problems for parents with regard to the impact on their work and childcare arrangements."
Although schools will still have to consult on changes, the method of consultation will be left for them to decide.
"We know from bitter experience with academy conversion that in too many cases, when it is left to schools, consultation simply is reduced to the lowest possible level of just informing those involved that changes will take place," Ms Keates said.
"Regulations like these were put in place for a good reason. No evidence has been presented as to why change is needed."
But Russell Hobby, general secretary of heads' union the NAHT, said he supported the changes.
"Longer school days can be one of the most powerful ways of bringing about school improvement. Carefully planned changes will be fine; heads are very aware of the needs of parents and their working lives."