Scottish First Minister Nicola Sturgeon on Monday demanded a new independence referendum in late 2018 or early 2019, once the terms of Britain's exit from the European Union have become clearer.

A vote that could rip apart the United Kingdom just months before Brexit would deepen the uncertainty surrounding the two-year process of leaving the EU after more than four decades.

"If Scotland is to have a real choice — when the terms of Brexit are known but before it is too late to choose our own course — then that choice must be offered between the autumn of next year, 2018, and the spring of 2019," Sturgeon told reporters.

Her demand comes just as British Prime Minister Theresa May is poised to launch the Brexit process, something opposed by most Scots in last June's vote on leaving the bloc.

Ultimately the U.K. parliament— where May commands a majority — makes the call on whether Scotland can hold a second referendum.

But if May refused to approve such a vote she could provoke a constitutional crisis while potentially stoking discord in Scotland.

While Sturgeon said the "door was still open" to talking to the U.K. government, she added that she was not expecting a change of tack by May's government over Brexit.