Tuesday’s strike by teachers in England would "harm children's education" and inconvenience parents, says Education Secretary Nicky Morgan.
The National Union of Teachers staged a strike over what it says is inadequate school funding.
NUT acting leader Kevin Courtney called for an increase in funding to meet rising costs for schools. But Mrs Morgan says the strike would "damage the profession's reputation in the eyes of the public".
The strike by one of the country's biggest classroom unions saw many schools closed and disrupted. The union also staged a series of regional rallies and marches in support of the strike. The education secretary on Monday said the walk-out was "unnecessary" and accused the union of "playing politics with children's futures".
Mrs Morgan said the government had protected school funding "when other areas of spending are having to be reduced".
Mr Courtney, the NUT's acting general secretary, said the union "wholeheartedly apologised" for disruption to parents, adding: "Teachers do not take strike action lightly."
"The problems facing education, however, are too great to be ignored and we know many parents share our concerns.
"The strike is about the underfunding of our schools and the negative impact it is having on children's education and teachers' terms and conditions."
Mr Courtney said it meant teaching posts were being cut or not filled and that class sizes would have to be increased. But Mrs Morgan, writing to Mr Courtney, said: "I am sorry that you have chosen the path of disruption over negotiation and discussion."
