Primary headteachers are to put their "heads above the parapet" in protest over the "gross underfunding" of the play-led foundation phase, TES Cymru can reveal.
In an unprecedented move, they have planned a day lobbying next Wednesday, leaving their deputies in charge.
So far heads from seven Welsh local authorities say they will protest and lobby for greater funding of the flagship FP, which becomes statutory for all under-fives this September.
The action, according to union leaders, follows the announcement by education minister Jane Hutt last week that an extra pound;5 million will be given for pilot FP schools - pound;10m short of what the Welsh Local Government Association (WLGA) says is needed.
Teaching unions say this is a conservative estimate and are outraged with the offering, which came after widespread condemnation of government funding.
Initial sums by the National Association of Head Teachers Cymru show shortfalls of up to pound;800,000 in some areas. In Bridgend, for example, additional funding covers less than 10 per cent of the amount schools will need.
Chris Howard, vice-president of the NAHT and head of Lewis School in Pengam, said lobbying heads had the support of governors and parents.
"All we have got are some vague promises. I've never known primary heads so willing to put their heads above the parapet," he said.
Mike Edwards, head of Maes-y-Coed Primary in Pontypridd, said he will lobby over lack of staff and outdoors facilities at his school.
"We'd love to do more outdoors work but it's hard to dig up the tarmac. We have to physically take the children out, and we need at least two staff to do that."
Teachers have serious concerns they will lack the funding to fulfil the 1:8 staff to children ratio advised for under-fives.
In a statement last week, Ms Hutt assured schools they would not be penalised during inspections. An Estyn spokesperson said inspectors would focus on how effectively schools use available resources, the quality of teaching and learning, and how well staff are used in the classroom during inspections.
"Estyn recognises the foundation phase will be at an introductory stage in many schools," said Ms Hutt.
But Mr Howard said: "If the minister was serious about allaying fears, then Estyn would be writing immediately to schools explaining how they would not be penalised."
Mr Evans, of the NUT Cymru, said: "If there's a school that hasn't been able to implement the FP fully, we would call upon Estyn to state quite clearly in the report that it is due to lack of funding."
Rex Phillips, regional organiser of the NASUWT Cymru, believes the shift of emphasis away from ratios could lead to a two-tier system between schools. He called for the FP to be phased in gradually.