A qualification that all aspiring heads have to complete is to be abolished in its current form, TES Cymru can reveal.
Recruitment for the National Professional Qualification for Headship (NPQH) has been frozen until a major revamp of the course has been completed following concerns that it is failing to deliver appropriate training.
The Assembly government said a new NPQH will be piloted from the autumn, but has not specified when the revised qualification will be ready.
The move follows sustained criticism of the two-year programme - described by schools inspectorate Estyn as "ineffective" and "outdated" - since became mandatory in 2005.
While the overhaul has been welcomed by teaching unions, concerns have been raised that without an immediate replacement the supply of heads could be damaged.
Gareth Jones, secretary of ASCL Cymru, said: "The NPQH has needed revision for some time, not just in content but in who is accepted on the programme.
"There's a potential gap in NPQH recruitment of 12 months. We would have concerns if that was extended any longer."
Recent cuts to money for teacher training have also prompted concerns over whether the revised NPQH will be properly funded.
"It's absolutely crucial to the future of education in Wales that we properly invest in leadership development," said Mr Jones.
Iwan Guy, acting director of NAHT Cymru, said: "Heads are already very concerned that there's no money for leadership training."
Estyn recently found that the NPQH had not served its purpose of producing heads within three years of passing, and that the supply of those who had completed the course far exceeded the demand for heads. In 2009, there were 1,271 holders, but only 532 of them had become heads.
Heads' unions said the revised programme should be less theoretical and more classroom-based, and the selection process should be tightened up.
In a letter to local authorities, the Assembly government said: "We need to ensure that we have a constant supply of teachers progressing to headship and will be maintaining a process of preparing candidates for this purpose. The revised arrangements are being considered and will be made public in due course."
An Assembly government spokesman added: "Funding for NPQH is not being withdrawn. Our current contract for delivery of the programme will end when the current cohort of candidates complete their programme.
"New arrangements for awarding NPQH are being developed and will be piloted in the autumn.
"The shortcomings of the current programme will be addressed through the revised arrangements, aligned with both the school effectiveness framework and Estyn's common inspection framework."