Rules permitting trans women to share female changing facilities and swim in a women-only pond are discriminatory and unlawful, the high court has heard.

The City of London Corporation is breaching equality legislation by allowing trans people to use the single-sex ponds on Hampstead Heath, according to a claim brought by the rights group Sex Matters. It is seeking permission to challenge the admission regulations.

Daniel Stilitz KC, for the City of London, said Sex Matters had “steamed in”, bringing a premature legal action at a time when its officials were actively consulting pond users on its entry rules.

Public bodies are redrafting their policies on single-sex spaces in response to the supreme court’s ruling in April that the legal definition of a woman is based on biological sex.

Hampstead Heath has three swimming ponds: one for women, one for men and a mixed area. People who identify as transgender, including those who describe themselves as gender fluid or non-binary, have been allowed to swim in the ponds of their preferred gender since at least 2019, in accordance with guidance from the Equality and Human Rights Commission.

In response to the supreme court ruling, the City of London concluded that the earlier EHRC guidance had misinterpreted the law. It withdrew its gender identity policy and embarked on a consultation process, gathering more than 38,000 responses to a review aimed at assessing how best to design future admission policies for the ponds, the court heard.

The results of the consultation are expected in January and a new policy is due to be published in March. In the interim, signs were erected outside the ladies’ ponds in July stating that the area could be used by women and those who identified as women. A sign outside the men’s pond explained that men and those who identified as trans men were welcome.

Sex Matters argues that this interim position, allowing swimmers entry into the ponds on the basis of self-identification of gender rather than biological sex, is unlawful. The EHRC has prepared new advice about how public bodies should balance competing demands for single-sex services and trans inclusion but it has not been published.

The charity included a list of complaints from female swimmers about the trans-inclusive policy in its legal submissions. The court heard details about the layout of the changing rooms in the ladies’ pond, which Tom Cross KC, acting for Sex Matters, said were mostly a communal space, with shared showers, involving full nudity.

Although the trans-inclusive policy applies to the men’s and ladies’ ponds, Cross argued that the admission rules treated “women less favourably than men because an individual woman is at greater risk of suffering the detriment of her privacy, dignity or safety being compromised than is an individual man”.

Stilitz KC said the application from Sex Matters was “busybodying” and “completely jumps the gun in circumstances where the admission arrangements for the ponds are currently under active review”.

He told the court that 38,000 people had “contributed to that exercise” and “the report on the results of that consultation are due in less than a month”, adding: “All the Corporation of London has done is maintain the status quo for a brief period before it can properly and thoroughly reconsider the admission arrangements”.

Segregating the ponds according to biological sex could be discriminatory to trans people, he said. “We have heard a lot about the risk of sex discrimination and nothing about the risk of gender reassignment discrimination, which is in play here,” he added.

Stilitz noted that, as well as communal showers, the women’s ponds had a room with a lock, where people could shower and change in private.

Mrs Justice Lieven said she would rule on whether Sex Matters should be given permission to have a full hearing to appeal against the City of London policy in January.