Updated 1.45pm with May's reaction
Scottish First Minister Nicola Sturgeon demanded a new Scottish independence referendum to be held in late 2018 or early 2019, once the terms of Britain's exit from the European Union have become clear.
"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, who heads Scotland's pro-independence devolved government, told reporters.
The prospect of an independence vote in Scotland that could rip apart the United Kingdom just months before an EU exit would add a tumultuous twist to Brexit with uncertain consequences for the world's fifth-largest economy.
British Prime Minister Theresa May is poised to launch the two-year process of taking the country out of the EU, something which was opposed by most Scots in last year's Brexit vote.
Sturgeon has called for Scotland to be allowed to strike its own deal with the EU but on Monday she said her efforts had hit a "brick wall" in London.
The results of the June 23 Brexit referendum called the future of the UK into question because England and Wales voted to leave the EU but Scotland and Northern Ireland voted to stay.
Sterling rose after Sturgeon said the earliest date for a new Scottish independence referendum was in the autumn of next year. British government bond prices fell.
Scots rejected independence by 55-45 percent in a referendum in September 2014, though the vote energised Scottish politics and support for Sturgeon's Scottish National Party (SNP) has surged since then.
Meanwhile, a spokesman for British Prime Minister Theresa May said a second Scottish independence referendum would be divisive and cause huge economic uncertainty at the worst possible time for Britain.
"Only a little over two years ago people in Scotland voted decisively to remain part of our United Kingdom in a referendum which the Scottish Government defined as a 'once in a generation' vote," the spokesman said in a statement.
"The evidence clearly shows that a majority of people in Scotland do not want a second independence referendum. Another referendum would be divisive and cause huge economic uncertainty at the worst possible time."