Members of the Scottish Family Party and supporters of the Gender Recognition Reform Bill protest alongside each other outside the Scottish Parliament.
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The UK government has blocked a new law intended to allow trans people in Scotland to change their legal gender without a medical diagnosis – a controversial move that has added fuel to the already highly emotional debate over Scottish independence.

Nicola Sturgeon, Scotland’s First Minister, called the intervention “a full-frontal attack on our democratically elected Scottish Parliament and its ability to make its own decisions on devolved matters,” in a post on Twitter on Monday.

Scottish Secretary Alister Jack – the minister representing Scotland in the UK government – announced Monday that Westminster had taken the highly unusual step of blocking the Scottish bill from becoming law because it was concerned about its impact on UK-wide equality laws – a justification that trans rights groups dismissed.

Here’s what you need to know:

What’s in the Scottish law ?

The law is designed to make it easier for people in Scotland to change their legal gender.

Under the current system, trans people must jump through a number of hoops to change the gender marker in their documents. They must have a medical diagnosis of gender dysphoria – a condition defined by the distress caused by the discrepancy between a person’s body and their gender identity – and prove that they’ve been living in their chosen gender for two years. They also need to be at least 18 years old.

The new rules would drop the medical diagnosis requirement, moving instead to self-determination. The waiting time would be cut from two years to six months, and the age limit lowered to 16.

Why did Scotland want to change the rules?

Campaigners have long argued that the current process is overly bureaucratic, expensive and intrusive. The Scottish government held two large public consultations on the issue and then proposed the new, simpler rules.

“We think that trans people should not have to go through a process that can be demeaning, intrusive, distressing and stressful in order to be legally recognized in their lived gender,” the government said when proposing the new rules.

In the end, a substantial majority of Scottish lawmakers from across the political spectrum voted for the change last month – the final tally was 86 for, 39 against.

What was the reaction in Scotland?

The bill sparked emotional reaction on both sides. The debate over the proposal was one of the longest, most heated in the history of the Scottish Parliament and the final vote had to be postponed after the session was interrupted by protesters shouting “shame on you” at the lawmakers.

Many human rights and equality organizations and campaigners welcomed the new rules, pointing to a growing number of democratic countries where self-determination is the norm.

The Equality Network, a leading Scottish LGBTI rights group, said that “after years of increasingly public prejudice against trans people, things have started to move forward.”

But the bill also attracted huge amounts of criticism, including from some lawmakers in the governing Scottish National Party (SNP) and “Harry Potter” author J.K. Rowling, who said the law could have a detrimental effect on the rights of women and girls.

Rowling and other opponents of the bill argued it would weaken the protection of spaces that are designed to make women feel safe, such as women-only shelters.

The Scottish government rejected that argument, saying the law didn’t change the rules on who can and cannot access single-sex spaces. It also said that experiences from countries that have made similar changes showed no adverse impact on other groups.

Campaigners agreed. “There are no downsides,” the LGBTQ campaign group Stonewall said. “For example when Ireland did it, nobody else was affected, except trans people who for the first time were able to have their gender recognised in a straightforward and empowering way by the state.”

Why is the UK government getting involved?

Scotland has a devolved government, established in 1999, which means that many, but not all, decisions are made at the Scottish Parliament in Holyrood, Edinburgh.

The Scots can pass their own laws on issues like healthcare, education and environment, while the UK Parliament in Westminster remains in charge of issues including defense, national security, migration and foreign policy.

The UK government can stop Scottish bills from becoming laws, but only in a few very specific cases – for example if it believes the Scottish bill would be incompatible with any international agreements or with the interests of defense and national security, or if it believes that the bill would clash with a UK-wide law on an issue that falls outside Scotland’s powers.

Under the rules that set out how Scotland is governed, London has four weeks to review a bill after it’s passed by Holyrood, after which it is sent to the King for Royal Assent, the last formal step that needs to happen before it becomes the law.

The deadline for intervention on the new gender bill was set to expire later this week.

But many in Scotland have accused the UK government of playing politics and blocking the new bill for political, rather than constitutional, reasons.

“Whatever your view on the bill, people vote in Scottish Parliament elections thinking they’re electing the MSPs who’ll represent [and] legislate for them on devolved matters,” said Emma Roddick, a Scottish Parliament lawmaker for the SNP. “[UK Prime Minister Rishi] Sunak has demonstrated what the SNP has said for decades: that’s only true as long as Westminster wants it. Two-thirds majority in favor of a bill within devolved competence – Tories we didn’t elect want to strike it down for political reasons.”

What is the UK government’s argument?

In his statement Monday, Jack argued that the bill would impact UK-wide equalities legislation.

The power over gender recognition laws is devolved to Scotland’s government, but legislation governing equality is set by Westminster.

“The Bill would have a significant impact on, amongst other things, GB-wide equalities matters in Scotland, England and Wales. I have concluded, therefore, that (blocking it) is the necessary and correct course of action.”

Speaking in the UK Parliament on Tuesday, Jack said he “had not taken the decision lightly.”

But advocates disagree. Rights group TransActual told CNN in a statement that it saw “no justification” for the UK government’s decision to block the bill over concern for UK-wide equality laws.

“There is no justification for this action by Scottish Secretary, Alister Jack. He will lose any case brought by the Scottish government, because the Equality Act is 100% independent of the Gender Recognition Act – and nothing in the Scottish Bill changes that,” Helen Belcher, the chair of TransActual, said in a statement.

“Trans people have never needed gender recognition to be protected by the Equality Act,” she added.

What’s next?

The Scottish government cannot overrule the decision. If it wants to take the legislation forward, it has two options: amend the bill and bring it back to the Scottish Parliament, or challenge the UK government in court.

Jack said Monday that he hoped to find “a constructive way forward” if the Scottish government decided to make changes to the law and present it again.

However, that seems unlikely. Sturgeon said in a tweet on Monday that her government would “defend the legislation and stand up for Scotland’s Parliament.”

There is no precedent, as the UK government has never before used its power to block Scottish legislation in this way.

“This is the first time the power has been exercised and I acknowledge this is a significant decision,” Jack told lawmakers Tuesday. “We should be clear that this is absolutely not about the United Kingdom government being able to veto Scottish Parliament legislation whenever it chooses, as some have implied.”

What’s the political background here?

For the past few years, the British government has leaned into the anti-trans culture wars debate in a bid to appeal to its traditional Conservative Party base and working-class voters in northern England who switched allegiance from the opposition Labour Party in the last election.

Former Prime Minister Boris Johnson’s government had stalled on a number of initiatives for the country’s LGBTQ community, including plans to make it easier for trans people to change their gender markers in England and Wales.

Questions remain whether it is a electorally viable strategy. Yet prior to becoming prime minister, one of the first pledges by Sunak during the Conservative Party’s leadership race in 2022 was protecting “women’s rights,” he wrote in a Twitter post.

The post linked to an article in which an unnamed Sunak ally told the Daily Mail that Sunak would create a manifesto opposing trans women competing in women’s sports and calling on schools “to be more careful in how they teach on issues of sex and gender.”

At the same time, tensions between London and Edinburgh over the issue of Scottish independence have been rising in recent years.

The SNP has already said it views the decision as another example of the UK government meddling in Scotland’s affairs.

Speaking in the UK Parliament on Tuesday, Anum Qaisar, an SNP lawmaker, said the decision to block the legislation was “shameful.”

“This decision sets a dangerous precedent where Westminster dictates and Holyrood must simply shut up and do as they are told,” she said.

When Scotland held a referendum in 2014, voters rejected the prospect of independence by 55% to 45% – but the political landscape has changed since then, mostly because of Brexit.

A majority of people in Scotland voted to remain in the European Union in the 2016 Brexit referendum, and the pro-independence Scottish National Party has argued that Scots were dragged out of the EU against their will. The SNP is pushing for a new independence vote but the UK government has said it will not agree to one.

Britain’s Supreme Court ruled in November that the Scottish government cannot unilaterally hold a second independence referendum.

CNN’s Tara John contributed to this report.