Wales is taking a gamble on a new education curriculum - and it will not work without teachers being given more time out of the classroom, a global expert has warned.
Dylan Wiliam, from University College London, said planned changes being introduced from 2022 could be successful - or a complete disaster.
The new curriculum will give teachers more say on what is taught, and how.
The Welsh Government said it was investing £40m to help schools prepare.
Prof Wiliam, emeritus professor of educational assessment, consults across the globe on formal education systems.
"There's a reasonable chance that this could be successful if the right things are done. And there's a really good chance that the whole thing is a disaster," he told BBC Radio Wales' Eye on Wales programme.
"It's still all to play for."
All primary schools and year seven pupils in Wales will begin the new curriculum in September 2022.
Subjects will be grouped under broader "areas of learning and experience".
The aim is to create more engaging and relevant lessons fit for the 21st Century.
The Welsh Government has recruited 164 pioneer schools to work on developing the curriculum framework.
The view from teachers
Rajvi Glasbrook, deputy head teacher at High Cross Primary School in Rogerstone near Newport, it was "equally a challenge as well as hugely exciting".
"There's an opportunity to empower an entire staff force," she said.
"Teachers will have the opportunity to think about the content they want to teach, but with that come challenges - namely time and resources."
Maesteg Comprehensive School is a pioneer in both teacher learning and curriculum development.
"We've had an opportunity to explore and unpick and question everything we do," Dale Duddridge, deputy head teacher said.
"At the centre of this is why do children come to school? I think teachers and schools need to be trusted to build the very best curriculum for the young people in front of them."
Education minister Kirsty Williams said she has already acted to ensure teachers can cope.
"I recognise that we need to get our professionals ready for this," she said.
"I have taken a very difficult decision on coming in to office to slow down the roll out of the curriculum to give all schools the professional time that they will need to engage in this process."
The changes will not be rolled out across the entire school setting for every year until 2026.
Prof Wiliam said introducing the curriculum changes was a "brave step".
But responding to the education minister, he added: "I don't think she's slowed it down anything like enough for this to be successful."
The Welsh Government is investing £24m in professional learning for teachers in Wales, on top of the £40m to implement the new curriculum.
Officials are also consulting on adding an extra teacher training day into the school calendar.
But Prof Wiliam says this is not enough.
"The curriculum doesn't give enough guidance and therefore the teachers have got to figure it out for themselves.
"If they haven't got the time, then they will carry on doing what they know how to do."
Eye on Wales is on BBC Radio Wales at 18:30 BST on Wednesday - or listen on BBC Sounds.