Flagship government policies to reduce class sizes in Scottish schools have been thrown into confusion after a landmark legal case.

A mother yesterday won a placing request appeal to send her daughter to the primary school of her choice, despite the fact West Lothian Council had previously blocked the move, claiming the class group was already full.

Although the authority agreed to the placing request before the full hearing at Linlithgow Sheriff Court, solicitors and education unions argue the case highlights the fact the only primary class size underpinned by the law is 30 - contained in regulations passed in 1999.

One interpretation is that any parent could get their child into any primary school as long as the maximum class size did not exceed 30, making a mockery of class size targets set by both the last Labour-run Executive and the current SNP Government.

If ministers decided to change the law to enforce smaller class sizes it would run completely counter to the spirit of the concordat or deal between councils and the Scottish Government, which merely calls for progress towards the target to be made "as quickly as possible".

Councils have been reducing pupil numbers in primary classes as a result of policy initiatives from successive Scottish Governments, aimed at driving up standards.

The former Labour-run Scottish Executive set a target of 25 for P1, while, under the concordat with local authorities, the new SNP Scottish Government has moved further, calling for class sizes of 18 in the first three years of primary. However, neither government legislated to enforce the new targets, with the executive choosing to issue a letter of guidance and the SNP relying on the concordat.

Last night, West Lothian Council , an SNP-run administration which has recently trumpeted its success in reducing class sizes, insisted the climbdown had come about because it expected an additional place to become available.

But Iain Nisbet, a solicitor with the Glasgow-based Govan Law Centre, who represented the family, believes the only logical explanation is that the council had legal advice that it could not win the case. "There is a strong suspicion they have looked at the case and realised that they could not win it because they now have a class size which is greater than 25," he said.

"It is a simple legal proposition that a guidance letter, or an agreement between the Scottish Government and local authorities, cannot over-rule statutory regulations.

"It is important that parents are aware the right to make a placing request to a school of their choice cannot be defeated by guidance or the terms of the concordat."

Ronnie Smith, general- secretary of the Educational Institute of Scotland, a national union which has campaigned for smaller class sizes, added: "We have serious concerns about the non-enforceability of these aspirational statements on class sizes.

"What we need is something more robust because, if this came to a judgment, the mechanism of concordat or guidance would not be a robust defence for a local authority to turn down a placing request. This is a major weakness."

John Stodter, general- secretary of ADES, which represents education directors, accepted that the current situation was confusing but argued that councils had the legal right to operate class size policies of their choosing.

"There is a legal opinion that it is up to local authorities to have a policy on class sizes and that policy would be founded on getting the best education for the children in the area," he said.

"In that case, the local authority is the legal entity and, therefore, it is up to it to decide what its policy is."

A spokesman for the Scottish Government said: "If this proves to be an issue it is, of course, something we will look at in future, but we do not expect it to become necessary."

A spokesman for West Lothian Council said: "It was more than likely this child would be admitted from the waiting list and, to put the matter beyond doubt and give the parent certainty, the court granted the application."