The British government announced on Monday it will use for the first time its veto power to stop a law passed by the Scottish devolved parliament which allows transgender people to self-declare their gender.

The Scottish parliament passed the controversial bill last month, amid some resistance within Scotland. It lowers the age for recognizing transgender people from 18 to 16 and scraps a medical certification requirement.

Secretary of State for Scotland Alister Jack however said on Monday that the administration of Prime Minister Rishi Sunak would block the bill, an unprecedented move since the Scottish parliament was established in 1999.

Why is London blocking the bill?

Jack said in a statement that the decision to block the bill was based on its legal implications for the entire Great Britain. He said it would particularly impact equalities matters.

"I have not taken this decision lightly," he said, adding that he acknowledged that the process transgender people go through to change their legal gender deserves respect, support and understanding.

If passed, the bill would set Scotland apart from England and Wales, where transgender people are not allowed in same-sex spaces. Opponents of the controversial law, in Scotland and beyond, argue it would grant predatory men access to female-only spaces.

Meanwhile, Scotland's First Minister Nicola Sturgeon had famously described the current process of sexual transitioning as "intrusive, traumatic and dehumanizing."

How could this veto escalate?

The law the Scottish parliament is attempting to pass has been implemented in other countries worldwide, including Ireland, Denmark and Argentina. The Spanish parliament also approved a similar bill last month.

Scotland's First Minister Sturgeon continued to defend the bill, threatening to challenge London's decision at the UK's top court.

In a tweet on Monday, Sturgeon, who faced unprecedented opposition within Scotland over the very same bill, described the veto as a "full-frontal attack on our democratically elected Scottish parliament and its ability to make its own decision on devolved matters."

The Scottish first minister warned that this precedent to veto a Scottish parliament bill could be "the first of many."

The potential legal showdown follows a tense phase in Scottish government relations with the UK government. It comes barely two months after the UK Supreme Court blocked efforts by Sturgeon's ruling Scottish National Party (SNP) to hold a new independence referendum.