(CNN) -- 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 it "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 Monday.Scottish Secretary Alister Jack earlier announced 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 dismiss.Here's what you need to know:What's in the Scottish law?Scotland passed a new law in December to make it easier for people 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.