Individual class sizes of up to 29 will now be permitted

The executive had vowed four years ago to cut S1 and S2 maths and English class sizes to a maximum of 20 in a drive to improve numeracy and literacy.

But class sizes of up to 29 will now be allowed if the overall average can be kept to 20, a move the executive claims will give teachers more flexibility.

The SNP said it showed the executive had failed to deliver on a key pledge.

The class size commitment formed part of Labour's coalition deal with the Liberal Democrats after the last Holyrood election.

If they wanted to meet this target they should have been recruiting more teachers much earlier Fiona Hyslop SNP education spokeswoman

However, in late 2004 the then education minister, Peter Peacock, said he had received "strong representations" from headteachers and local authorities seeking flexibility on class size reductions.

Councils have now been told that individual class sizes of up to 29 will be permitted, provided the average across the school year group in those subjects is 20.

Education Minister Hugh Henry said the executive position had not changed.

"We have made it clear that we are committed to reducing class sizes in S1 and S2 English and maths to a maximum of 20," he said.

"We have invested in the resources that will allow every school to do this by training more teachers, funding salaries and investing in school buildings where more classrooms are needed."

However, he said some councils and headteachers wanted flexibility over maximum class sizes.

'Clear breach'

Mr Henry explained: "That could mean some S1 and S2 English and maths classes would be smaller than 20 and others - with more able pupils - might be slightly larger.

"In some exceptional circumstances, where the headteacher is convinced that pupils will benefit, and where proper consultation has taken place with parents, classes could be slightly larger than 20."

However, Fiona Hyslop, the SNP's education spokeswoman, said: "We had our suspicions when they started talking about flexibility but now we can see for the first time that this blanket policy has been dropped.

"If they wanted to meet this target they should have been recruiting more teachers much earlier."

Ronnie Smith, general secretary of the EIS teaching union, told BBC Radio Scotland: "I think it's a very clear breach of promise.

"Everybody understood what the promise was, and that was a maximum of 20.

"An average of 20 is not the same as a maximum, so it's clear they've reneged on the promise made to the teachers and parents of Scotland."