Michael Gove has called for longer school days and suggested that teachers should also be happy with longer terms, on the same day he announced schools should be allowed to sack staff after a term.
The education secretary said "we're all in favour" of extending the school day, as well as potentially also cutting short summer holidays.
Asked about how this would impact on teachers, he told ITV's Daybreak programme: "If you love your job then there is, I think, absolutely nothing to complain about in making sure you have more of a chance to do it well."
Such a move would benefit poorer children, he suggested, as "poorer children from poorer homes lose learning over the long summer holidays".
A number of schools had taken up the initiative and it appeared to have positive results, he said.
But Chris Keates, general secretary of the NASUWT union, said teachers were entitled to have a good work-life balance.
"Has anybody ever tried to put themselves in the position of teachers?" she asked.
"Every single day someone is telling them what to teach, how to teach, when to teach and how long they should teach, with no thought for the conditions of service and no thought for the challenges that the job entails.
"Everybody, no matter how much they love their job, is entitled to have a proper work-life balance and nobody needs that more than teachers," she added.
Christine Blower, general secretary of the National Union of Teachers (NUT), said there was evidence that teachers are already working 60 hours a week, not all of which is in the classroom.
She said it was hard to see what extra time was available.