The Conservatives have promised to rewrite the Equality Act so that protections it enshrines on the basis of a person’s sex apply only to their biological sex.
Prime Minister Rishi Sunak said the "safety of women and girls" meant the "current confusion around definitions of sex and gender" cannot be allowed to continue.
The Tories say their election pledge will make it simpler for service providers for women and girls, such as those running sessions for domestic abuse victims, to stop biological males from taking part.
Labour's shadow defence secretary John Healey told OceanNewsUK the change was not needed adding that it was "an election distraction from the really core issues that matter to people ".
He said the Equality Act "already protects single sex spaces for biological women" but acknowledged there was a need for "clearer guidance", something he said Labour would produce.
Liberal Democrat deputy leader Daisy Cooper told the OceanNewsUK: "If there are any examples of where a public service needs to have further guidance we're happy to issue that guidance but let's not pick apart legislation that reflects the hard-fought and hard-won protections given to women and trans women.”
The proposals would also see UK government decisions on gender reassignment applied across the UK, including Scotland.
Scottish National Party leader John Swinney accused the Conservatives of being "on the war path against the Scottish Parliament and the devolved government in Scotland".
Under the Equality Act 2010, it is illegal to discriminate against anyone because of "protected characteristics".
These are a set of identifying traits that are protected by law and include age, disability, religion, race, sex and sexual orientation among others.
Some interpret sex in this context as referring strictly to biological sex, while others believe it also applies to people with a gender recognition certificate; a legal document that allows someone to change the legal sex on their birth certificate.
The Conservatives want to change the act to apply to biological sex, and say single-sex spaces, such as public toilets and hospital wards, and services such as rape crisis centresdo not have to be open to those who are biologically male but identify as female.
The party says that since the Equality Act was introduced in 2010, it has not kept up with "evolving interpretations" of sex and gender.
The Conservatives say the policy is a matter of principle but it is also an issue the party has used to attack its opponents.
Questions on transgender care and identification have created divisions within the Labour Party and the SNP.
The move is not without political risk for the Conservatives though. The Equality Act was passed by Labour in 2010 - meaning the Conservatives have had their entire 14 years in office to fix those areas they now regard as problems with it.
Mr Sunak said: “The safety of women and girls is too important to allow the current confusion around definitions of sex and gender to persist.
“The Conservatives believe that making this change in law will enhance protections in a way that respects the privacy and dignity of everyone in society."
Critics have, in the past, accused the Conservatives of seeking to stoke divisions and demonise transgender people in the search for votes.
However, minister for women and equalities Kemi Badenoch said she was "not trying to stop trans people from living their lives as they wish".
She told OceanNewsUK: "We are trying to create a chilling effect for those men who are predators who are exploiting trans people and exploiting laws we have put in place to protect trans people."
Ms Badenoch was not clear about what paperwork would be sufficient for a transwoman to access women-only spaces such as rape crisis centres.
Asked if birth certificates would be accepted - including ones amended after someone's gender change had been recognised in law - she said this was "not a paperwork issue".
“What is happening at the moment is that people come to these centres and they are visibly of a different sex.
"If a rape crisis centre decides that it wants to allow a trans woman with a gender recognition certificate, they will be able to do so. If they choose not to, then they can’t be sued for that."
The Conservatives have also committed to making gender reassignment a reserved issue; meaning it would be uniform policy across the UK nations.
This comes after the UK government last year intervened to stop Scotland enacting its Gender Recognition Reform Bill.
Ms Badenoch said in the Times: "We are one United Kingdom and it is impracticable for gender recognition regimes to vary in different parts of the country. So, we will also legislate to establish that gender recognition is a reserved matter."
The Equality Act 2010 already allows service providers to exclude certain groups if doing so is considered a "proportionate means" of achieving a "legitimate aim".
In 2022, the UK's equality watchdog, The Equality and Human Rights Commission (EHRC), published guidance for service providers wishing to keep spaces - such as bathrooms - single-sex, including examples of when they can legally exclude certain groups in order to do so.
One such example was around access to domestic abuse refuges, with the EHRC advising it would be legal to bar trans women from the refuge, if female survivors indicated they felt uncomfortable.
In 2022, Judge Lady Haldane concluded the definition of sex in the act was "not limited to biological or birth sex" after a group of campaigners launched a legal case around gender balance on public boards in Scotland.
The Conservatives argue the "ambiguity" of the law means single-sex service providers are often "vulnerable" to legal challenges, and that women and girls' safety is at risk.
The proposed law change would apply whether or not the person has a gender recognition certificate.
It's not the first time the Conservatives have spoken about amending the Equality Act.
In 2023, Ms Badenoch wrote to the EHRC for advice on the impact of doing so.
The EHRC's chair Baroness Kishwer Falkner said it would give clarity in a "polarised and contentious" area, but also warned a change could be "more ambiguous" than the current definition of sex in relation to equal pay and sex discrimination.
She said any changes to the law would need detailed analysis of possible disadvantages for trans men and women in these areas.
