The rights of transgender women to use a women-only pond in north London have been acknowledged in a new policy.
Swimmers on Hampstead Heath will be able to use ponds "aligning with their gender identity", the City of London Corporation's (CoLC) has said.
Admission will be granted on a case-by-case basis under the policy.
However, Stonewall said the 2010 Equality Act already protected trans people from being discriminated against when accessing services.
Stonewall director Laura Russell said it was "not a new rule".
She added: "Trans people's right to use single-sex spaces, regardless of whether they have legal gender recognition, has been the law for nearly a decade."
But feminist campaigner Amy Desir, who uses Kenwood Ladies' Pond at Hampstead, called the policy "absolutely disgusting".
Ms Desir, from campaign group ReSisters UK, said the policy "disproportionately discriminates against young women" and was "open to abuse".
She added: "Under the policy any man can self-identify and declare themselves a woman.
"The CoLC is deliberately misusing the Equalities Act and basing the policy on a biased survey."
Writer and trans-commentator Jane Fae said she was "entirely unsurprised" by the CoLC policy.
She said: "All they have done is endorse the law as it stands. If they had done the opposite they would have been taken to court."
Joanne Conaghan, a Professor of Law at Bristol University, said: "Legislation governing the rights of trans people is complicated because the law relating to gender recognition and the rules governing discrimination on grounds of gender reassignment do not neatly align.
"In particular, the protections accorded to trans people under the Equality Act 2010 is wider than the right to gender recognition conferred by the Gender Recognition Act 2004.
"There are limited situations in which transgender people may be denied access to sex-specific services under the Equalities Act 2010, but the City of London's policy is correct to allow trans people a presumption of inclusivity to use ponds that align with their gender identity."
A consultation on attitudes to gender identity held last year received nearly 40,000 responses.
CoLC said 65% of the valid respondents to last year's survey favoured ensuring trans people did not suffer discrimination.
But 46% of the total responses to the consultation were disregarded as invalid on the basis that those respondents did not answer any questions, other than to identify themselves and declare the reason for their interest in the survey.
Last year, female activists demonstrated against the right of trans women to use the women's pond by using the men's pond.
The Kenwood Ladies' Pond Association said it welcomed the decision.
A spokeswoman said: "The Ladies' Pond is a single sex space and the KLPA is committed to helping to create there an inclusive environment for all women, including transgender women, which is free from discrimination, harassment or victimisation."
Edward Lord, chair of the CoLC establishment committee, said: "This policy will ensure our public services do not discriminate against trans people.
"All communities should be fully respected, and equality and basic human rights upheld."