A flagship early-years policy looks set to be delayed because of concerns about funding, staffing and changes needed to the law.
The new play-based foundation phase for three to seven-year-olds, being piloted across Wales, was to be extended to all children aged up to five from next September.
The initiative, which could result in children not having formal lessons until they are seven, has been widely welcomed by pilot schools and playgroups.
But headteachers have raised concerns about funding to ensure pupils in schools enjoy the same adult-to-child ratios (1:8) as are required in the pre-school sector, and for providing adequate indoor and outdoor space.
There have also been problems finding enough qualified early- years support staff, particularly in the Welsh-medium sector.
Jane Davidson, education and lifelong learning minister, is expected to announce a revised timetable at next week's National Primary Trust conference in Rhondda Cynon Taf.
Earlier this month, she told heads that improving early-years provision was a key way to prevent disadvantage in later life, by giving children a "flying start".
But in response to questions, she said: "We were a little ambitious in the timescale we originally set. We will not be going for a full roll-out in September 2006, not least because we must have all the legislative requirements in place and the right timetable for training up all the necessary staff.
Heledd Hayes, education officer with the National Union of Teachers Cymru, which supports the changes, said she was not surprised by the delay:
"Education through play for very young children is a very skilled task to perform. There is no purpose to the foundation phase unless we have the staff who can deliver."