Improving literacy among pupils is one of the Welsh government's top priorities. But fears have been raised over the ability of new teachers to deliver, following criticism of trainees' spelling and punctuation.
The weaknesses were highlighted in the first inspection of one of Wales' three new regional teacher training institutions, which were set up as part of grand plans to improve standards among those joining the profession.
Estyn inspectors who visited the South West Wales Centre of Teacher Education - the first teacher training institution to be scrutinised under a new inspection framework - described its work as only "adequate", partly because of trainees' poor literacy.
A minority did not have "secure enough literacy skills", the report said. "They make errors of punctuation and spelling in their written work and in the classroom, and a very few do not model oral language accurately."
While all students are screened for literacy levels on entry, trainees did not always access programmes to improve their skills and the centre did not monitor their progress well enough, the inspectors added.
The findings come as the Welsh government renews its focus on improving literacy among pupils and raising the bar for those wanting to become teachers. It recently consulted on proposals for new functional literacy and numeracy tests to be a prerequisite for joining training courses.
In a similar development, the Westminster government is also ramping up entry standards for new teachers. It announced plans last week to introduce tougher entrance exams, with candidates expected to pass tests in English and maths equivalent to a GCSE grade B.
The South West Wales Centre is one of three regional centres established in 2009 after a review called for greater collaboration between training institutions. It was formally launched last year as a partnership between the University of Wales Trinity Saint David in Carmarthen and Swansea Metropolitan University.
Philip Dixon, director of teaching union ATL Cymru, called the judgement of the Estyn inspectors "disappointing".
"The report, the first on such an institution, highlights a number of concerns, but literacy is obviously a key one," Dr Dixon said. "With the renewed importance that the Welsh government is now attaching to literacy and numeracy skills, it is clear that the centre needs to up its game and challenge some of the students more.
"It is vital that those who emerge from colleges are enabled to deliver the new national literacy and numeracy frameworks," he added.
In a self-evaluation report last October, the centre realised that a minority of its undergraduate and postgraduate trainees needed additional support in their literacy skills. Nevertheless, inspectors still found issues when they visited in May this year, and last week their report recommended that the centre improve its trainees' skills and their ability to apply them in a teaching context.
Dr Russell Grigg, head of the centre, said he welcomed the challenges raised by the report. "We have strengths in our current performance and these outweigh areas for improvement," he said. "Colleagues know, for example, that while the majority of our trainees are good language role models, a minority do not have good enough literacy skills, in Welsh and in English."
The timing of the report is not helpful for the centre, as the government is currently focusing on the future of initial teacher training (ITT) in Wales.
Education minister Leighton Andrews has commissioned a review of ITT by Professor Ralph Tabberer, former director of the Teacher Training Agency and former director general for schools at the Department for Children, Schools and Families. Professor Tabberer's report, due next spring, will focus on the quality and consistency of teaching, training and assessment in ITT, as well as course structure and the coverage of issues.
Changes to the minimum entry requirements for ITT courses for all trainees are due to be made in the 2014-15 academic year. Entrants will need a minimum GCSE grade B in English and maths or equivalent, rather than a grade C.
A Welsh government spokesman said: "It is essential that our teachers have the skills they need to succeed in their job."
New statistics show that less than half of the students who qualify as teachers in Wales go on to teach in Wales.
Only 47.5 per cent of those who qualified through the PGCE route and 45.9 per cent who qualified after a first degree went on to a teaching post in Wales in 2010-11.
The percentages are similar to the previous two years.
Aled Roberts, the Liberal Democrat education spokesman, called on the Welsh government to review provision so that PGCE places more accurately reflect demand.