Some schools in Wales will have to close as a result of teacher strikes, according to the education minister.
National Education Union (NEU) members are expected to strike on 1 February on the first of four days of action.
Jeremy Miles said the exact number of schools "isn't clear" but head teachers were calculating provision based on numbers of union members in schools.
Councils wanted to give "a week's notice to parents of what that will mean in schools", he said.
It still remains unclear whether any online teaching will be provided on strike days.