The former PM, known for her 49 days in Downing Street, is looking to present an amendment to the Health and Equality Acts on Wednesday to the Commons.
The bill claims to protect single sex spaces by amending the Equality Act so “sex” is unambiguously referred to as biological sex.
It also suggests preventing formal state – or school – recognition of social transitioning for anyone under 18.
Truss’s bill wants to make sure anyone under 18 cannot undergo body-altering hormone therapies which are used to treat gender dysphoria, either – that includes puberty blockers and cross-sex hormones.
A spokesperson from the transgender youth charity Mermaids, told HuffPost UK: “We are disgusted by this blatant attempt to roll back hard-won rights for the trans community.”
They continued: “Trans people, young and old, deserve the same autonomy and respect as cis people to live their lives authentically and access the services they need.
“We know firsthand from the young people we support how transformative it is to their mental and physical wellbeing, when they are in an environment where their pronouns are respected, and they have the space to explore their identity.”
Truss’s plans to redefine sex were also slammed as it would risk “removing the rights and protections trans people have fought tirelessly for, following decades of discrimination and marginalisation.”
“Sex rights and trans rights are not at odds with each other and do not need to be treated as such,” Mermaids concluded.
While promoting her bill, Truss said: “It has become clear that the law as it stands is not providing sufficient protection for young people or indeed adult women.
“Getting this law onto the Statute Book would not only safeguard the rights of women and protect the wellbeing of our children, but it would also be a significant victory for common sense.”
The bill is being sponsored by former home secretary Priti Patel, along with nine other Tory backbenchers and one DUP MP.
This is not the first time Truss has campaigned on such issues.
Speaking to The Times in September, Truss pushed for her successor Rishi Sunak to publish guidance “stating that schools should be clear about biological sex and not officially sanction social transition”.
At the time, she said: “There needs to be clarity about biological sex in school, particularly for toilets and changing areas, which should be single sex spaces.”
The former PM also claimed transitioning is “not a neutral act” and so should not be officially sanctioned for those under 18.
“I do not believe that this contravenes the Equality Act but if there are any legal issues, the government should legislate,” she said.
Previous reports suggest a complete ban on pupils transitioning would actually be illegal under the 2010 Equality Act.
Truss also oversaw the exclusion of trans people from a planned government ban on conversion therapy when she was equalities minister.
She dropped bills for gender “self-identification”, too. This would have allowed the trans community to officially change their gender on certain documents without going through a medical approval process.
Shortly before she got into No.10, Truss said she did not think trans women are women, and said: “Under-18s shouldn’t be able to make irreversible decisions about their own future.”
Truss’s bill comes at a time when the government has already been criticised over its anti-transgender rhetoric.
Charities told HuffPost UK earlier this year that its policies had left people “scared, tired and alone”.