some of Wales's best headteacher potential is being lost because the criteria for the now compulsoryJNational Professional Qualification for Headship (NPQH) is too narrow, it has been claimed.
Estyn, the inspection body,Jalso found poor practice in some appointments, fuelling earlier criticism that gaining headships in Wales can be about "who you know, not what you know".
Inspectors said that the criteria used to select candidates for the NPQH excludes some applicants, such as advisers and teacher trainers, who are fit for the job.
In a report on the initial impact of the qualification, Estyn recommends the Welsh Assembly government looks at how far the selection procedure is turning away talent by requiring course providers to report on present limitations.
But an Assembly spokesperson said this week that aspiring headteachers did not have to meet all the requirements of the national standards for headship in order to apply for the NPQH.
He added: "There is no expectation on applicants to demonstrate that they are able to meet the (national standards for headship) in full until they have completed the NPQH programme."
Unions this week agreed that the criteria for the NPQH qualification are too exclusive.
Criticism that not enough is being done in Wales to groom future heads has already been made over the past few months. Slightly more than 300 candidates have applied for the NPQH since it became mandatory for heads in September 2006.
But John Evans, the president of the National Association of Headteachers Cymru, said: "Unless people can get a place on the course to obtain this qualification, they are precluded from doing the job. This applies to acting heads who are unable to apply for the post even though they may have been doing the job for years.
"There must be more leeway," he added.
Inspectors found that the new selection procedure, based on revised national standards for heads in Wales, was "fairer and more efficient" than before, but they also noted that the revised standards were excluding some candidates "who might well be suited to headship".
In the report, Estyn queries why a quarter of vacant posts in Wales last year were filled by acting heads, and calls for an investigation by the Assembly government.
Elsewhere, the report says both urban schools and Welsh-medium schools were experiencing problems in getting suitable heads because they received far fewer applications.
While most headship vacancies attract between six and 15 applicants, this drops to just two or three in some large urban schools and Welsh-medium schools, says Estyn.
The agency recommends that the Assembly should implement a mentoring programme for senior teachers to raise their interest and confidence in applying for headships in such schools.
There is also concern that some local authorities do not check whether a candidate has the new NPQH qualification.
"In a very small number of cases, there is poor practice in fair selection procedures such as the use of unsuitable selection criteria," the report adds.
This included "the acceptance of unsolicited and unsubstantiated evidence such as the reputation of the applicant in the LEA".
A spokesperson for the General Teaching Council for Wales said: "The report recognises, as we do, that more needs to be done to address the shortage of applicants for headteacher posts in Welsh-medium schools."