Scotland should hold a new referendum on independence from the United Kingdom by 2021 if Britain leaves the European Union, the Scottish leader Nicola Sturgeon said Wednesday, even as she acknowledged that she lacked the power to make that happen on her own.

Scots voted against independence about 54 percent to 46 percent in a 2014 referendum billed as a once-in-a-generation poll.

In 2016, the United Kingdom as a whole voted to leave the European Union, but people in Scotland voted strongly to remain.

Ms. Sturgeon, Scotland’s first minister and the leader of the pro-independence Scottish National Party, has argued that Brexit changes the situation because Scotland should not be dragged out of the 28-member European Union against its will.