J.K. Rowling has been criticised for supporting a woman fired for saying biological sex is 'immutable'.

Harry Potter author J.K. Rowling has found herself in the midst of a major Twitter storm after lending her public support to a British woman who recently lost her job as a result of sharing her beliefs about a person’s sex — that it is an immutable, biological fact.

Dress however you please.

Call yourself whatever you like.

Sleep with any consenting adult who’ll have you.

Live your best life in peace and security.

But force women out of their jobs for stating that sex is real? #IStandWithMaya #ThisIsNotADrill — J.K. Rowling (@jk_rowling) December 19, 2019

“Dress however you please. Call yourself whatever you like. Sleep with any consenting adult who’ll have you. Live your best life in peace and security. But force women out of their jobs for stating that sex is real?” Rowling wrote on Thursday morning.

The tweet quickly racked up over 41,000 likes and more than 17,000 comments.

She concluded her tweet with the hashtags #ThisIsNotADrill and #IStandWithMaya, the latter referring to the case of Maya Forstater, the woman at the centre of the controversy.

The controversy over the definition of sex

View photos Maya Forstater (left) lost an appeal over anti-trans language. Photo: Facebook/maya.forstater More

Maya is out of a job as a thinktank researcher after an employment tribunal ruled that her opinions on biological sex were “absolutist”.

The 45-year-old was a visiting fellow at the Centre for Global Development (CGD), which campaigns against poverty and inequality.

Her contract was reportedly not renewed in March after a dispute over her tweets — declaring that transgender women cannot change their biological sex and voicing opposition to government proposals to reform the Gender Recognition Act, which would allow people to self-identify their gender.

On Wednesday, Judge James Taylor ruled that the researcher’s views did “not have the protected characteristic of philosophical belief”.

Responding to the verdict on Wednesday, she tweeted: "I struggle to express the shock and disbelief I feel at reading this judgment."

“My belief as I set out in my witness statement is that sex is a biological fact, and is immutable,” she further explained, on a CrowdJustice fundraising page.

“ There are two sexes, male and female. Men and boys are male. Women and girls are female. It is impossible to change sex. These were until very recently understood as basic facts of life by almost everyone. “

“As I said at my tribunal I will as a matter of courtesy use preferred pronouns and I support human rights. Everyone should be free to express themselves, to break free of gender stereotypes and to live free of violence, harassment and discrimination. But this does not require removing people’s freedom to speak about objective reality, or to discuss proposed changes to law and to government policies clearly.”

Forstater was clearly thrilled by Rowling’s much-buzzed-about response to the ruling.

OMG! We are all crying. This is all i wanted for Christmas https://t.co/5ucPGUHIxY — Maya Forstater (@MForstater) December 19, 2019

Some furious over ‘transphobic’ sentiment

But Rowling’s stance also stoked plenty of anger, with transgender activists and other supporters firing back at the author, declaring her what is known, derogatorily, as a TERF — transgender exclusionary radical feminists.

Since the tweet, the phrase “JK Rowling is a TERF” has gained serious traction on Twitter.

Charlotte Clymer, press secretary for the non-profit LGBTQ Human Rights Campaign, weighed in on the controversy on Twitter: "In fact, the World Health Organization — among countless other medical authorities — validate trans people in their authentic gender identity. It is quite clear you don't understand the first thing about the trans community or the science at play. This is heartbreaking."