the inspection body's recommendation that the assembly government should send out additional guidance to teachers comes after some were said to be allowing their pupils to "play aimlessly".
It also follows on from a report by academics, who unearthed a fall in literacy standards when they carried out an evaluation of the "learning through play" strategy on behalf of the government nine months ago.
At a conference last December to launch the report, Professor Iram Siraj Blatchford, of London's Institute of Education and co-author of the evaluation paper, spoke of a "significant decline" in literacy, language and communication standards, and called for more hands-on teaching guidance in the FP, without a return to desk-based learning.
The Foundation Phase Pilots report, released by Estyn this week, also revealed a lack of assessment channels and a resistance to outdoor play at some schools. These flaws will cast doubt on the future of the hugely popular learning technique, which is due to be introduced to all under-5s in Wales from September 2008.
But Susan Lewis, chief inspector of education and training in Wales, was keen to accentuate the positive: "Estyn inspectors have found that almost all the staff in pilot schools are enthusiastic about the FP and show high levels of commitment to its success," she said this week.
The report also reveals that boys, who loved the outdoor play, were benefiting greatly and additional funding had helped improve pupil-staff ratios in maintained settings. And one-third of staff reported improved listening and speaking skills.
But examination of the full report reveals deep uncertainty and a lack of clarity among teaching staff.
Although Estyn reports that teachers' confidence has been boosted by the strategy, they are not so assured over funding levels, concluding that these have not met either "need or expectation".
The report also asserts that not all pupils are being assessed correctly, with some teachers unsure how to go about their observations.
Meanwhile, teachers in a quarter of schools were found to be "unsure or unconvinced" of the benefits of outdoor play. Some also restricted it for fear of a parental backlash for taking children out in the cold.
Estyn's main recommendations include issuing guidance defining what is meant by "child-led", "adult-led" and "outdoor learning", and setting up an all Wales system to measure pupils' progress.
An Assembly government spokesperson said: "This report confirms much of what we learned from the monitoring and evaluation report last December.
"The recommendations in that report and that of Estyn are being addressed through our delivery of the Building the Foundation Phase action plan. Key elements in that plan are the preparation of an FP national training pack, to be supported by a pound;2 million training budget and a series of revised curriculum guidance materials.
"Funding for the FP has grown steadily from a budget of pound;0.5m in September 2004, when the pilot started, to over pound;10m this year."
Leading article, page 16