The number of junior school classes with more than 30 pupils has increased by almost 40 per cent in the past year, new figures reveal.
Assembly government statistics show that 236 junior classes across the country had more than 30 pupils at the start of term, compared with 172 last year.
Education minister Leighton Andrews called the increase "disappointing", and urged schools and local authorities to work together to keep numbers down.
But headteacher Terry Williams said primary schools are finding it increasingly tough to keep junior class sizes down because of their tightening budgets.
Mr Williams, head of Litchard Primary in Bridgend and president of heads' union NAHT Cymru, said: "This year I have a junior class of 34 pupils because I can't afford to split them into two classes. We have put a full- time support person in there so the pupils don't lose out educationally.
"I can't see the situation getting much better nationally. If schools haven't got the funds, they have got to play with the cards they are dealt.
"But if the funding is not there they need support and the flexibility to manage it as best they can in the interest of pupils - there mustn't be too much top-down pressure to conform to targets."
Since 2001, the Assembly government has set a target of ensuring junior classes contain 30 pupils or fewer.
This September 4.7 per cent of junior classes had more than 30 pupils, compared with 3.4 per cent last year and 25 per cent in 1999.
At infant level there has been strict legislation since 2001 limiting classes to 30 pupils or fewer.
But last year there was widespread concern when figures revealed that 2.5 per cent of infant classes had more than 30 pupils - up from just 1 per cent between 2004 and 2007.
This September that figure fell slightly to 2.4 per cent, meaning that only 97 classes had more than 30 pupils.
Of those, 85 classes had legally permitted exceptions, such as pupils accepted outside the normal round of admissions.
The Assembly government said it is working with the relevant local education authorities and schools to ensure that the 12 remaining classes comply with the legislative limit by the start of next school term in January 2011.
Leighton Andrews said: "We are committed to ensuring local education authorities and schools carry on maintaining primary class sizes of 30 or fewer. I am sure that this is a target which will continue to receive support from parents and teachers.
"We want all our young people to receive the very best start to life and will assist schools and local authorities to ensure they provide them with opportunities to reach their full potential."
"The continuing roll-out of our pioneering foundation phase will ensure that all infant-aged children benefit from even greater levels of small group working and individual attention."
- Original headline: Close call: 30-plus junior school classes soar by nearly 40%