The NASUWT teaching union said its members in the city in South Wales had voted overwhelmingly to take part in industrial action.

More than half of Swansea's secondary schools will close for the day while a number of others are partially closed.

Swansea Council said it was "extremely disappointed" by the union's decision and refuted claims of "employer abuse, victimisation or failure to comply with employment agreements".

The union said its members "do not take strike action lightly" and said it "will not hesitate to take further action" if necessary.

Neil Butler, the NASUWT's national official for Wales, said teachers "must be confident" they will be supported when they intervene in violent incidents "to keep themselves and their pupils safe".

"Swansea Council has not followed its own policies correctly and has failed to honour collective agreements it has with the NASUWT," he said.

"The NASUWT will not tolerate any abuse of its members by employers and this strike action today shows members are resolute in standing up for their rights and protections in the workplace."

The NASUWT's national executive member for Swansea, Helen Johns, said talks with Swansea Council had been "ineffective in securing what we [the union] seek".

"The breach was triggered by nothing but the local authority's unfounded intervention in a school process," she added.

The disagreement is claimed to be over an incident involving a male teacher who pulled two male students apart when they were engaged in a physical fight.

According to the union, the teacher was disciplined but later exonerated on appeal before a second appeal led to his dismissal.

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A spokesperson for Swansea Council said strike action would "only serve to deny children of educational opportunities".

"We refute any allegation of employer abuse, victimisation or failure to comply with employment agreements with teachers which they will need to substantiate.

"The council has been speaking regularly with the union and has met four of the five requirements that they put to us to end the dispute."

The council said it could not comment on the specifics of the individual case for legal reasons.