The U.K. government is threatening to block a new Scottish law that imposes a self-identification system for people who want to change their sex, The Guardian writes.

In a significant escalation that brings tensions over transgender rights into the constitutional arena, Westminster has made clear that it will consider the nuclear option of blocking the bill for royal approval, a move that would infuriate change advocates and nationalists.

The Scottish government promised to vigorously fight any such interference.

First Minister Nicola Sturgeon called Sunak and his cabinet negative Democrats after the Supreme Court ruled in October that Scotland could not legally hold another independence referendum without Westminster approval. The ruling led to an increase in independence votes over the past month.

The Scottish government has welcomed legislation to simplify the gender reassignment process, as well as to expand a streamlined system for 16- and 17-year-olds to obtain a gender reassignment certificate (GRC).

Minister for Women and Equality Kemi Badenoch, who met with her Scottish government counterpart Shona Robison to discuss the bill, said after the vote that the Scottish government has not fully considered the implications of its bill, particularly on the lives of women and girls.

The U.K. government is now considering provisions that could be revised.

The intervention followed briefings by the U.K. government ahead of the vote that the new law would create legal chaos and lead to gender tourism across the border with England.