The Scottish Government can legally change the definition of 'woman' to include transgender people, a judge has ruled.
The definition of 'woman' was broadened in legislation designed to increase female representation on public boards, meaning it would no longer be defined as a "female of any age".
For Women Scotland (FWS) brought a judicial review against the changes, arguing that they breach equality laws.
"This leaves us with a definition that includes some men, while, remarkably, excluding some women. This cannot be allowed to stand," the group said.
"We are concerned about the potential implications beyond Scotland, as, if it becomes established that devolved legislatures can amend key terms in the Equality Act via other pieces of legislation, then other countries within the UK may follow and the Equality Act may be eroded."
But judge Lady Wise rejected this claim on Tuesday and ruled that the Scottish Government could lawfully legislate on aspects of the Equality Act.
"It should be understood at the outset that the case does not form part of the policy debate about transgender rights, a highly contentious policy issue to which this decision cannot properly contribute," she said.
"At its core, this litigation is concerned with whether certain statutory provisions were beyond the legislative competence of the Scottish parliament.
"While I record certain statements that were made about Scottish ministers' policy or position on transgender rights, that matter was at best tangential to the central dispute and has had no bearing on the decision that I have made."
FWS told The Scotsman it was disappointed by the ruling and would be consulting its lawyers.
"At a time where the endemic nature of the discrimination and violence women experience on the basis of sex is more apparent than ever, it will come as a shock to many women in Scotland that the Scottish government can redefine what it means to be a woman in law, so that it is little more than a name on a utilities bill," it said.
"We will be following with interest what candidates and parties have to say in the upcoming election campaign, and they can certainly expect to be asked whether they support women's rights and the use of single-sex exemptions under the Equality Act 2010, ensuring that women have the right to single-sex spaces such as changing rooms, refuges and rape crisis counselling."