Thousands of children in England and Wales face disruption to their education as teachers strike over pay, pensions and conditions.
Members of the National Union of Teachers (NUT) will take part in the action, leaving many schools closed to some or all pupils.
Union leaders said the national walkout was a "last resort" but would go ahead.
The Department for Education (DfE) said the NUT was disrupting parents' lives and holding back children's education.
The union said the strike action on Wednesday was covered by two ballots held in May 2011 and June 2012.
In total, 92% of members voted in favour of strike action, with a turnout of 40%. The NUT has 326,930 members in England and Wales.
(BBC)