Welsh teaching unions are threatening to take legal action against local authorities that dock pay from teachers who were stranded abroad by the volcanic ash cloud that closed British airspace for several days.
In a joint letter issued this week, the unions said they would consider salary deductions unlawful and would support members who took the matter to employment tribunals.
While the majority of Wales' teachers were unaffected by the travel chaos because their Easter break ended the week before the ash cloud hit, hundreds of staff from the six local authorities whose holidays ended on April 16 found themselves stranded abroad.
Both Swansea and Pembrokeshire have already indicated that while staff on school trips would be paid, those who failed to return from private holidays would have to take unpaid leave.
A Swansea Council spokesman said discussions were taking place with unions about how the policy was applied.
A Pembrokeshire County Council spokesman said: "The standard position is that thesy will be granted authorised leave but will not get paid.
"However, the authority is prepared to judge each case on its merit."
Secretary of NUT Cymru David Evans said: "We take the view that these deductions are unlawful. We oppose them and if necessary will take action to protect our members' rights."
Rex Philips, Wales organiser of the NASUWT, called the situation "an absolute disgrace", and accused councils of using it as an opportunity to save money at their teachers' expense.
Gareth Jones, secretary of ASCL Cymru, said good will should be shown towards teachers in light of the "very unusual circumstances".
Jane Morris, director of Governors Wales, said pay should not be deducted and authorities should consider alternative approaches.
In Swansea it is estimated that up to 150 teachers were affected by the flight ban.
Wayne Newton, head of Morriston comprehensive in Swansea, said four of his teachers and two support staff were stranded abroad on private holidays.
Although all made it back to the UK by last weekend, each now faces losing up to five days' pay.
Mr Newton said: "There's a lot of unrest about this among Swansea heads, but as it's a council directive we can't act independently.
"I personally think a little bit more flexibility and understanding could have been given."
In Carmarthenshire, heads and governors are being advised by the council to consider alternatives to deducting pay, including working out of school hours to make up the time.
A spokesman said: "Where alternative arrangements have been made no pay deduction will be necessary."
Gwynedd Council said that teachers had less flexibility in their working arrangements than other council staff, and it would co-operate with the teaching unions to determine the scope for staff to make up any lost time.
Original paper headline: Fury erupts as teachers face docked pay for delayed returns to classroom