Members of the National Association of Schoolmasters Union of Women Teachers in Kent and the Medway towns will vote in a county-wide ballot, over whether to work strictly to contract.

Contracts say teachers can be asked to cover for absent colleagues and vacancies only in “exceptional” circumstances. But unions claim their members are being exploited by having to cover regularly.

Kent County Council officials say their schools have 267 vacancies, most of which are filled with temporary staff.

Allan Craig, NASUWT national executive member for Kent and the Medway Towns, warned teachers: “Your school may not have a problem but ... that happy state of affairs may not last much longer.”

Elsewhere, members of the both the NASUWT and the National Union of Teachers in Greater London and Doncaster voted this week on whether to stop providing cover.

More than300,000 members of the two unions are already boycotting meetings and administration work in protest at escalating workloads and the action is beginning to bite. A hotline run by the Secondary Heads Association has had some 500 calls from heads struggling to cope. Teachers are refusing to attend meetings, reports to parents have been reduced to one a year and heads are increasingly being forced to teach.

Russell Clarke, SHA deputy general secretary, said: “A number of important routines are being disrupted. A great deal of work to get parents involved in their children’s schooling is being lost.”

At a Midlands infants’ school, staff had to be rounded up to attend a meeting called by the head to announce her resignation. Meanwhile the head of a large secondary in the south is teaching 15 hours a week because staff are refusing to provide cover.

Employers have threatened to dock the pay of teachers who refuse to cover but Mr Clarke said heads were reluctant to take disciplinary action.