The Welsh NHS bill will “go through the roof” without regular PE in schools, Baroness Tanni Grey-Thompson has said.
Due to be introduced in 2022, the draft curriculum does not specify a set amount of physical activity every week.
The former Paralympic gold medallist, who sits in the House of Lords, said: “If time is not carved out to do it, it will just disappear.”
The Welsh Government said the curriculum “takes into consideration the importance of physical activity”.
The curriculum leaves it open for individual schools to decide on PE lessons.
Baroness Grey-Thompson said she was concerned they would disappear off the curriculum in some schools, causing problems for the NHS in 15 to 20 years’ time due to people being unfit.
“Because PE is difficult to teach, it’s one of those things that I really worry will slip away because there are other things that are easier to do,” she told BBC Wales Live.
“If sport is not explicitly mentioned, it will just drop off. Whatever the meaning and the intention, it won’t have the same priority.
“We won’t see the problem right now, we’ll see it 15 or 20 years down the line when the NHS bill goes through the roof because we have a generation of young adults who are just not fit enough to be healthy.”
Baroness Grey-Thompson’s concern is echoed by a group of AMs, who recommended schools be required by law to provide at least two hours of PE every week.
The Health, Social Care and Sport Committee said the change was vital to tackle a public health crisis, with more than a quarter of four and five-year-olds starting school in Wales last year classed as obese or overweight.
But supporters of the new curriculum said it would lead to children being more active and healthy as, for the first time, the wellbeing of pupils would be placed at the heart of school life.
In 2013, Baroness Grey-Thompson chaired a group which examined PE in Welsh schools. Its main recommendation – that PE be made a core subject with the same status as maths, English, Welsh and science – was not implemented.
Education expert Prof Graham Donaldson was then commissioned by the Welsh Government to draw up a new curriculum.
The first draft was published last month after consultations with schools and is made up of six areas of learning and experience – including wellbeing, which incorporates physical activity alongside topics like sexuality, relationships and healthy eating.
Gethin Mon Thomas from GwE, the school improvement service for north Wales, said the new curriculum would deliver better outcomes than the plan proposed by Baroness Grey-Thompson.
“We have an area of learning and experience which occupies an increased amount of time on the school timetable,” he said.
“The curriculum oozes opportunities for sport to be used as a vehicle to support learning.
“We will actually put sport and physical activity in a position where it is truly valued in society, rather than just being something that small groups of individuals benefit from.”
Sport Wales, the organisation responsible for promoting sport and physical activity in Wales, supports the new curriculum but its chief executive Sarah Powell said its success would stand or fall on the training teachers receive.
“The essential thing is to build up the confidence, motivation and skills of teachers to be able to deliver a high quality curriculum. But if we don’t see that, then this is a curriculum that doesn’t actually deliver the changes that we need to see.”
The Welsh Government said the curriculum supported children’s development, ensuring that they grow up to be healthy and confident individuals.
- There will be more on this story on Wales Live, BBC One Wales, at 22:30 BST on 12 June