Teachers in Northern Ireland have been warned there may be "contractual implications" if they take industrial action.
It could lead to part of their salary being deducted for breaching their contract.
That is according to a letter from the teaching employers to school principals, seen by BBC News NI.
Members of the NASUWT union have been told they risk losing pay if they are not carrying out all contracted duties.
But all of the other main teaching unions in Northern Ireland - the INTO, UTU, NAHT and NEU - are also now balloting their members on industrial action.
In February, teaching unions in Northern Ireland rejected a two-year pay offer from the employers as "inadequate".
