Primary schools will struggle to keep infant class sizes below the maximum legal limit of 30 pupils because of growing demands on their budgets, according to a headteachers' union.
The warning comes amid concerns that success in cutting class sizes over the past decade is already beginning to falter.
Figures released this week reveal that 2.5 per cent of infant classes have more than 30 pupils - up from just 1 per cent between 2004 and 2007.
Anna Brychan, director of heads' union NAHT Cymru, said: "Our members are increasingly telling us how much pressure they are under in this regard. We feel very strongly that this is something that needs to be looked at closely in the next few years. It's becoming increasingly financially difficult for schools to stick to these limits."
The law limiting infant class sizes to 30 was introduced in 2001. It had followed concerns that numbers were too high, with 6.4 per cent of classes having more than 30 children in 1999.
This week's figures show that of the 3,991 infant classes in Wales, 99 had more than 30 pupils in the annual September count, compared with 68 the same time last year.
After taking into account legally permitted exceptions, such as pupils accepted outside the normal round of admissions, 16 classes were over the limit, one more than last year.
The Assembly government said it was working to make sure that those classes complied with the legal limit by the start of the next school term in January.
Education minister Jane Hutt said: "The Assembly government is committed to ensuring that local education authorities and schools carry on maintaining primary class sizes of 30 or less - a target, I am sure, that will continue to receive support from both parents and teachers."
Ms Hutt urged schools and councils to work together to eradicate larger class sizes.
Some schools have been forced to get creative to meet the 30-pupil limit by reorganising and introducing mixed-age classes. Since 2001 the Assembly government has also set a target of ensuring junior classes contain 30 pupils or fewer.
This September there were 59 junior classes over the 30-pupil limit, without permitted exceptions, compared with 74 last year.
There are no laws governing class sizes in Welsh secondary schools. Gareth Jones, secretary of the Association of School and College Leaders Cymru, said colleagues in secondary schools would welcome the funding for similar targets in their sector.