Schools are at risk of becoming "narrowly focused on personal attainment" by playing down their spiritual side, according to one of Britain's most senior clerics.
Writing exclusively in this week's TES, the Archbishop of Wales, Dr Barry Morgan, has attacked a new law allowing post-16s in the principality to opt out of daily collective worship.
In a strongly worded missive he warns that the move - bringing Wales in line with England, which scrapped compulsory worship for post-16s in 2007 - would be the "thin end of the wedge", marginalising religion in schools.
"Collective worship has been branded as something that young people grow out of by the age of 16, at precisely the time when it might be the best way of feeding both their minds and their hearts," he said.
"Without a clear recognition of a spiritual dimension, schools run the risk of becoming narrowly focused on personal attainment."
Sixth-formers could previously only withdraw with permission from their parents.
Dr Morgan expressed particular concern about the impact of the law on younger children.
"The value of this daily focal point for a school community will hardly be acknowledged by pupils lower down the school if those whom they look up to in the sixth form choose not to attend."
But Neil Foden, head of Ysgol Friars in Bangor, said this did not reflect the reality of modern secondaries.
"When the legislation was first drafted the idea of collective worship was posited on the primary school model where the whole school comes together," he said.
"But most schools don't have room to physically accommodate every pupil in one place."
He said it would have only a "minimal impact" on his school.