A charity has won a court bid to continue a legal challenge over trans people being allowed to use single-sex facilities at Hampstead Heath's swimming ponds.
Sex Matters appealed a ruling it could not go ahead with High Court action against the City of London Corporation, which manages the 790-acre open space in north London.
The charity sought to contest the corporation's policy of allowing trans people to use the facilities at the ponds for the gender with which they identify - claiming that this approach amounts to sex discrimination.
Currently, biological women and transgender women are allowed to use the ladies' pond, while men and transgender men are admitted to the men's pond.
A consultation was launched to review the access of trans women following a Supreme Court ruling last April that a person's legal sex is the one they were biologically assigned at birth.
Mrs Justice Lieven said in an earlier verdict in January that the 'appropriate forum' for the claim challenging the policy and brought by Sex Matters was the county court rather than the High Court.
Sex Matters last month said it had applied for permission to challenge the decision at London's Court of Appeal on the basis that the judge was 'wrong in law'.
And in an order on Monday, judge Lady Justice Elisabeth Laing said the legal challenge could go ahead and would return to the High Court.
The charity Sex Matters has now been given permission to press ahead at London's High Court with a legal challenge against the Corporation of London's current policy at the heath.
She said: 'The judge did not engage with the merits of the grounds for judicial review except at the end of her judgment.'
Lady Justice Laing later said it was 'arguable' that parts of the previous judgment were wrong, including a decision that the case had been brought too late.
Sex Matters CEO Maya Forstater said the charity was 'delighted' by the decision.
She said the ruling 'confirmed that as a specialist charity with objections focused on the sound administration of the law in relation to sex, Sex Matters has standing to take this case'.
She added: 'This case will have implications for single sex provision all over the country.'
'The Government's failure to publish updated regulatory guidance following last year's Supreme Court judgment has given businesses and service providers an excuse to continue flouting the law on single sex provision as clarified by the country's most senior judges.
'A ruling in our favour by the High Court would demonstrate that failing to comply with the law has consequences.'
A new hearing will be set by the High Court in London at a later date.
In written submissions in support of the appeal bid, Tom Cross, for Sex Matters, said that Mrs Justice Lieven was 'plainly wrong' to refuse to allow the legal challenge to proceed.
The barrister continued that it was 'contrary to the public interest, the rule of law, and the sound administration of justice' for the issues raised in the claim not to be considered at the High Court.
Mr Cross told a hearing in December that the current rules governing access to the ponds treat an individual woman 'less favourably' than an individual man, and that the corporation should change its policy in light of the Supreme Court ruling.
But Daniel Stilitz, for the Corporation of London, said that the claim was 'premature' as the body was consulting on a new policy.
The corporation has said results show almost 90 per cent of respondents backed trans-inclusive access to the ponds, after more than 38,000 people took part in the consultation over a period of two months.
Of those, 84 per cent of respondents to the consultation had swum at the bathing ponds and 74 per cent lived in London.
Six options were considered for the Kenwood Ladies, Highgate Men's and Hampstead mixed ponds, with 86 per cent of respondents supporting the existing trans-inclusive access arrangements.
Some 86 per cent also opposed introducing strict single sex access, 90 per cent of people rejected requiring trans swimmers to use separate changing rooms or have separate swimming sessions and 66 per cent opposed making all ponds mixed sex.
A spokesperson for the City of London Corporation, said in response to the latest court ruling: 'We note the decision and will continue to contest this case vigorously.
'These continuing legal challenges require significant time and resources, diverting funds away from managing Hampstead Heath as a registered charity and providing high-quality services for the public.
'The results of our recent public consultation showed that a clear majority of respondents - including a high number of regular pond users - supported continuing the current, trans-inclusive arrangements.
'Members of the relevant City Corporation committees will consider these findings alongside legal duties, equality impacts, safeguarding responsibilities, and operational considerations when making a final decision about the future access arrangements of the bathing ponds on Hampstead Heath in due course.'
Critics of the current policy say the ponds' current system undermines women's rights to privacy and dignity.
A protest was held last May, with a group of women - some wearing false beards, wigs and moustaches - trying to enter the male-only space in defiance of the rules while claiming to self-identify as male.
They were met by security guards and police who locked a gate to prevent them from entering while insisting that protests were not allowed.
Amy Desir, a gender-critical campaigner who organised the protest, said at the time: 'If you allow trans women or men who identify as women into the ladies only pond then why can't us women, who identify as men, just for today, be allowed into the male only pond?'
She told the Daily Mail: 'We are not transphobic, we are pro-women and want to maintain safe spaces for women.
'The Corporation's self-identity policy is driving women away from the women's pond. They don't feel safe there anymore because of transwomen or predatory men being let in because they say they're a woman.
'The whole situation is ridiculous. People who in the eyes of the law are not women are being allowed to use the women's pond and that is disgraceful.'
The City Corporation has insisted it is 'compliant with existing UK law'.
City of London Corporation policy chairman Chris Hayward previously said in January about the consultation results: 'The volume and tone of responses we received demonstrate very clearly just how much the ponds are valued as calm, safe, welcoming community spaces for all to enjoy.
'While we've been clear that the consultation was not a referendum, carefully reviewing the findings from it will form an important part of our wider decision-making process which we will communicate clearly to the public in months ahead.
'It's important that we take time to ensure future access arrangements are fair, lawful, evidence-based and, crucially, respectful to those who use swimming ponds.'
Hampstead Heath forms part of the 11,000 acres of open space across London and south-east owned and managed by City of London Corporation.