Most primary schools will be able to accommodate the Scottish Executive's commitment to reducing class sizes, teachers said yesterday.

Under plans by the SNP- led executive, class sizes in the first three years of primary school are to be cut to just 18, starting with schools in deprived areas.

Although no date has been set by which the target is to be delivered, the proposals sparked concern that popular schools would struggle to meet the commitments unless they turned some parents away.

Gordon Smith, headteacher of Jordanhill Primary in Glasgow, warned at the weekend the implications of such a reduction could mean some families not getting in.

"I can see us telling people, You are not going to Jordanhill, you are going to Drumchapel'. That would have an interesting reaction among parents," he said.

Hugh Henry, Labour's education spokesman, warned in a debate in Holyrood that the cuts could mean extra classrooms having to be built. "Will we see a rash of portable cabins across Scotland as the SNP moves towards an ill-thought out policy?" he asked.

However, Ronnie Smith, general secretary of the Education Institute of Scotland (EIS), which has campaigned for smaller classes, said the majority of schools would cope. He pointed to recent executive figures which indicate there are around 180,000 empty places in primary schools and 33% of primaries are less than 60% occupied.

In addition, population trends suggest the number of primary pupils is projected to fall from 390,000 two years ago to 366,000 by 2010 and 352,000 by 2019.

"To highlight schools where there is pressure on places at the expense of the majority where there is not is an unbalanced position which does not take into account the great benefits this policy would bring to pupils across Scotland," said Mr Smith.

Mr Smith said the decline in pupil numbers, coupled with the largest programme yet of school building and refurbishment should ensure no local authority had difficulties.

Pat Watters, president of Cosla, , which oversees all of Scotland's local authorities, said: "Reducing class sizes presents practical challenges for councils but we want to overcome these by working constructively with government."

Greg Dempster, general secretary of the Association of Head Teachers and Deputy Head Teachers in Scotland, said the phased approach, targeting deprived areas, was welcome and "practical".