An pledge to reduce primary class sizes by the SNP was dealt a blow yesterday when new research claimed that evidence on the benefits of the policy is inconclusive.

A committee of MSPs was told that the research from an education centre at Glasgow University found that reductions in class sizes could be costly and that other factors such as quality of teaching and a child's home background were just as important.

But the experts did say that smaller classes could help some pupils, with younger children and less able students most likely to benefit.

The Scottish government refined the pledge made in the SNP manifesto for last year's Holyrood election to reduce class sizes to 18 in Primary 1 to 3. Now, ministers say, they will “work towards” the target.

The researchers said that from the evidence of studies in the US and England, there was little change in pupil performance between class sizes of 18 and 25. The Educational Institute of Scotland (EIS), the country's largest teaching union, wants a significant reduction in class sizes and is urging MSPs on Holyrood's Lifelong Learning Committee to back a petition signed by 80,000 people.

In evidence to the committee, David Drever, the EIS vice-president, said that the union wanted the cuts extended across the board.

Mr Drever claimed that where there was evidence of any setback in a pupil's attainment, it was because youngsters had moved from a small class size into a much a larger one.

Meanwhile, the Scottish Conservatives called yesterday for a £10million fund to be set up to give every school pupil in Scotland one week of adventure education.

Some schools send youngsters on courses where they can take part in activities such as kayaking, climbing and abseiling. The Tories claim that these programmes can also help pupils learn valuable team-building skills and boost their self-confidence.

The idea has won the backing of Gavin Hastings, the former Scottish rugby captain.