In an unprecedented move, the UK Government has confirmed it will block Scotland's gender reform bill from becoming law. 

The Gender Recognition Reform (Scotland) Bill has been approved by MSPs in December after a lengthy series of debates, but for the first time UK Government ministers have stopped the legislation from coming into force. 

Prime Minister Rishi Sunak and Scottish Secretary Alistair Jack have confirmed intentions to trigger Section 35 of the Scotland Act. 

First Minister Nicola Sturgeon has said it was a "full-frontal attack on our democratically elected Scottish Parliament" and SNP's social justice secretary Shona Robison deemed it an "outrageous decision". 

READ MORE: UK Government confirms move to block Scotland's gender reforms

But what is Section 35 of the Scotland Act? 

Most simply, section 35 allows the UK Government to stop the Presiding officer from submitting a Bill for royal assent - the final step before the proposed legislation can become law.  

It is included in the Scotland Act which established the devolution settlement largely as we know it today in 1998.

However, the power to intervene is granted only under certain and limited conditions. 

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 legally take action if it sees a bill as making modifications to the law as “it applies to reserved matters”.

Under this scenario, there would once again need to be "reasonable grounds" which makes the Secretary of State believe the legislation would have "adverse effects on the operation of the law as it applies to reserved matters." 

This would allow for a bill to be blocked if it clashes with prevailing UK Government policy as it applies to reserved matters, even if the proposed legislation falls within the powers granted to Holyrood.

Why is Section 35 controversial? 

The order has never been triggered in the nearly 25 years of Scottish devolution and has been criticised as an undemocratic intervention by London to override the decision of elected representatives in Holyrood.

The move is likely to launch a constitutional clash between Mr Sunak’s Government and the Ms Sturgeon's administration.

What is the legislation that triggered the decision?

Suggestions of a Section 35 order were raised as soon as the Gender Recognition Reform (Scotland) Bill was passed by MSPs in December. 

The bill, which went through by 86 votes to 39 and three days of lengthy debates, will remove the need for trans people to require a gender dysphoria diagnosis to obtain a gender recognition certificate (GRC).

It would also speed up the process of applying for a GRC as well as lowering the age limit to 16.

The legislation was backed by all political parties at Holyrood except the Scottish Conservatives.

READ MORE: What you need to know about Scotland's gender recognition legislation

Why is Section 35 being used? 

Scottish Secretary Alister Jack has set out his intention to use Section 35 of the Scotland Act to pause the Holyrood reforms over concerns it impacts on UK-wide legislation.

He claimed that the gender reform legislation would have an "adverse impact on the operation of Great Britain-wide equalities legislation". 

Mr Jack, who has written to both Ms Sturgeon and Holyrood’s Presiding Officer Alison Johnstone about the matter, insisted that “transgender people who are going through the process to change their legal sex deserve our respect, support and understanding”.

He stated: “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.”

What will the Scottish Government do?

Nicola Sturgeon has said she will “absolutely, robustly and rigorously” defend the legislation. 

Social justice secretary Shona Robison added: "This is a dark day for trans rights and a dark day for democracy in the UK.

“As the First Minister said, this is a political decision that is more in keeping with UK Government’s contempt for devolution and the Scottish Parliament.

“We have always been clear that the Bill does not impact on the Equality Act. The UK Government had multiple opportunities to comment during the extensive consultation on the Bill and during its passage and we are confident that the legislation as passed is within legislative competence of the Scottish Parliament.

“We will examine the reasons which the UK Government give for their order once we have them, but take whatever steps we can to ensure that the democratic will of the Scottish Parliament is not frustrated.”