The assembly government's decision to scrap a key fund for continuing professional development (CPD) has come under fire after inspectors found that it improves pupils' results.
An Estyn report published this week said the individual CPD programme, administered by the General Teaching Council for Wales (GTCW) since 2001, has had a positive impact on the classroom and wider development of schools.
The pound;3 million annual programme has been used by more than 30,000 teachers since 2001.
But the government ended funding in April and launched a review of CPD, causing an outcry.
GTCW chief executive Gary Brace said Estyn's report underlined the importance of the programme and the urgent need to fund training.
"This is a really positive acknowledgement by Estyn of the major benefits the programme has brought for the profession and for many pupils," he said. "It proves CPD is not there just to benefit teachers.
"But the tragedy is that this is the very programme that has been closed down."
Mr Brace said he was disappointed that the report did not recommend reinstating the programme, which he said was needed as a "matter of urgency". It was the only fund to which teachers, rather than schools, could apply directly.
"I'm really concerned that teachers have the opportunity to benefit from this funding," said Mr Brace. "I'm not concerned who controls it, I'm concerned with seeing it restored.
"I wouldn't be surprised if teachers start to become much more vocal about their need to be supported."
The programme allowed individual teachers to apply individually for money to aid their further training and development. Funding ranged from pound;650 for shorter training to pound;5,000 for longer sabbaticals.
Estyn's report said that projects often served as a springboard for teachers to undertake further study.
It said they are most effective when backed by heads. The majority are linked to their school's development goals so contribute to improving overall school performance.
However, inspectors found that many teachers do not effectively monitor or evaluate their own professional development.
The report made only one recommendation, that the government should fund materials to help teachers evaluate the impact of CPD on their teaching and raising achievement.
Chief inspector Ann Keane said: "These findings are very encouraging in supporting the improvement in standards of education and training in Wales. It clearly demonstrates the impact that professional development in teachers can make."
An Assembly government spokesman said: "We are now working with stakeholders to develop a clear model for all educational practitioners. The main focus will be on improving classroom practice and developing leadership skills."