It will be physically impossible to deliver the foundation phase (FP) to under-fives without one member of staff for every eight children.
That was the verdict of teachers and learning support assistants attending one of the premier events of the primary sector calendar in Cardiff last week.
Many teachers at the annual conference of the Primary Network Cymru felt the play-led strategy would "fall apart" if the 1:8 ratio was not met.
It now appears certain that schools will not be able to afford - nor be able to find - enough trained staff for the FP's roll-out for under-fives this September.
Pilot schools that have been well-funded up till now fear they will have to make drastic cutbacks on roll-out, resulting in falling standards.
Ian Forbes, head of St Mary's Church in Wales School in Blaenau Gwent, said children's education would suffer.
He said: "It's going to be impossible to do it without those levels of funding.
"The ratios were the selling points right from the start, and it needs those qualified, dedicated professionals."
Teachers are threatening to stage boycotts amid claims of significant underfunding, threats of redundancies and staff shortages.
But teachers at St Mary's Church, an FP pilot school, spoke of the consequences based on their own experiences.
They said that extra staff were essential for supervision and structuring activities - as well as weekly planning and preparation - which are fundamental to the scheme's ethos.
The school's teachers have completely restructured the way lessons are run, with all staff working as a team.
Teachers and learning support assistants work with small groups of children, often outdoors, and all are involved in planning classes which are labour, time and resource-intensive.
"Without the people and the necessary ratios it just isn't going to work," said nursery teacher Claire Wright.
Mr Forbes said pilot schools, such as St Mary's, are proving how well the FP can work.
"We have achieved everything that they wanted us to do," he said.
"We thought Wales was going to be at the forefront of education here, and we were confident that the funding would continue."
The Association of Directors of Education in Wales estimates that of the pound;107 million promised by the Assembly government over three years, only pound;41m is additional funding.
Mr Forbes said: "The question is, why didn't the government do the figures four years ago when it started considering the amount of work and millions of pounds that have gone into it?"
Kirsty Williams, education spokesperson for the Liberal Democrats, stressed the need for a more transparent funding system.
She claimed some local authorities are prioritising funding to pilot schools at the expense of non-pilots, and that within schools some leaders are shifting resources from other classes.
"If we do not a get a grip on these issues, the benefits for children and teachers that have been clearly demonstrated by the pilots, with pupils who are more engaged and who have better speaking and listening skills, and hugely increased levels of staff confidence, will not be realised across Wales," said Ms Williams.
Experts recommended a 1:8 adult-to-child ratio for under-fives, rising to 1:10 with a fully qualified teacher in class. However, the ratio is not statutory, and many schools now fear they will be way off the recommended number of staff.
Speaking at the conference last week, education minister Jane Hutt tried to reassure headteachers that the foundation phase was a priority.
Following the threat of a headteacher boycott of the FP, she is holding crisis talks with the Welsh Local Government Association and local authorities.
"I hope that assures you that I'm putting it at the top my agenda," she told delegates.