WESTMINSTER has chosen the “nuclear option” button by moving to block Scotland's gender reform law, in a move sure to trigger a major constitutional showdown between London and Edinburgh.

The UK Government has deployed a little-known weapon in its legal arsenal, under the Scotland Act, to block the Scottish Government’s gender recognition reforms, which MSPs passed overwhelmingly just before Christmas.

It has issued a Section 35 order – something that has never been done up until now.

What is a Section 35 order?

National readers will be familiar with a Section 30 order – the part of the Scotland Act which temporarily gives the Edinburgh parliament the power to pass laws on reserved matters, such as holding the 2014 independence referendum.

A Section 35 order allows the UK Government to block Scottish Parliament legislation under certain circumstances.

READ MORE: 'Outrage' if UK blocks Scotland's gender reforms from becoming law, says Nicola Sturgeon 

It is a power afforded by the Scotland Act – the Westminster legislation which created the Scottish Parliament in 1998 – but it can only be used in limited circumstances.

It works by allowing the Scottish Secretary, a post currently held by Alister Jack (below), to stop Holyrood’s Presiding Officer (the Scottish equivalent of the Speaker) from referring the bill to the monarch for royal assent.

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Royal assent is crucial for bills to become law. Any piece of proposed legislation that has passed all stages in the law-making process must undergo the formality of gaining royal assent – the monarch’s agreement to make the bill an act.

Under Section 35 the UK Government can legally intervene to block royal assent for a bill that a Secretary of State “has reasonable grounds to believe would be incompatible with any international obligations or the interests of defence or national security”.

The Government can also act if it believes a bill would change the law as “it applies to reserved matters”.

READ MORE: Blocking gender bill would be 'gift' to independence movement, says Tory MSP

In this case, ministers in London must have “reasonable grounds” to believe that any proposed legislation would have an “adverse effect on the operation of the law as it applies to reserved matters”.

This means that a bill can be blocked by Westminster if the Scottish law was believed to be at risk of conflicting with UK law – even if the proposed legislation fell within the powers granted to Holyrood.

The bill specifically mentions its intentions to not modify the UK Equality Act of 2010 – meaning the UK Government will need to prove that they have concerns the Scottish bill will affect UK legislation.

Why is the Government using this power?

MSPs last month passed the Gender Recognition Reform (Scotland) Bill by 86 votes to 39, approving reforms which would make it easier for trans people to change their gender on official documents by simplifying the process of obtaining a Gender Recognition Certificate (GRC).

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The bill will also allow 16 and 17-year-olds to apply for a GRC for the first time and would reduce the amount of time a person has to live in their new gender before they can be granted the document.

It will also remove the need for a medical diagnosis to get a GRC – something trans campaigners have long argued was humiliating and invasive.

READ MORE: Keir Starmer faces backlash for contradicting Scottish Labour on gender reform

The UK Government immediately raised concerns, claiming the bill could have ramifications for UK equality law and other pieces of UK-wide legislation.

Downing Street has also expressed concerns the bill could pose problems for single-sex spaces and that the proposed law would remove the checks and balances involved in gaining a GRC.

For its part, the Scottish Government has said it would have no impact on the exemptions or the wider Equality Act – which is reserved to Westminster.

In a statement issued on Monday evening, the Scottish Secretary said: "After thorough and careful consideration of all the relevant advice and the policy implications, I am concerned that this legislation would have an adverse impact on the operation of Great Britain-wide equalities legislation. 

“Transgender people who are going through the process to change their legal sex deserve our respect, support and understanding. My decision today is about the legislation’s consequences for the operation of GB-wide equalities protections and other reserved matters. 

“I have not taken this decision lightly. 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 this is the necessary and correct course of action. 

“If the Scottish Government chooses to bring an amended Bill back for reconsideration in the Scottish Parliament, I hope we can work together to find a constructive way forward that both respects devolution and the operation of UK Parliament legislation. 

Why is Section 35 controversial?

The power has never been used before and to government critics it is an undemocratic intervention by London to override the decision of elected representatives in Edinburgh, effectively undermining devolution.

The move will unleash a constitutional war between Sunak’s government and the Scottish administration.

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Tory MSP Jamie Greene (above), a supporter of the bill, said prior to Monday's announcement that the UK Government blocking the law would be a gift to the independence movement and the FM has said it would be an outrage.

What will the Scottish Government do if Section 35 is used?

The Scottish Government has said it will “vigorously” contest any attempt to block the bill from becoming law.

This would probably result in legal action against Sunak’s Government, leading to what could be another protracted court battle between Edinburgh and London.

LGBTQ+ charity Stonewall has called such an outcome a “nuclear option” which could turn a debate about transgender rights and gender recognition into a broader row over the devolution settlement.