Theresa May | Pool photo by Stefan Rousseau/Getty Images May rejects Scottish government plan for independence vote British prime minister’s stance puts her on collision course with Nicola Sturgeon.

British Prime Minister Theresa May on Thursday rejected a Scottish government plan for a second referendum on Scottish independence.

She was responding to a call from Scottish First Minister Nicola Sturgeon for a referendum to be held between the fall of 2018 and spring of 2019, around the time the U.K.’s Brexit negotiations are scheduled to come to an end.

May made clear she was not prepared to consider any request for a referendum at the moment and Ruth Davidson, the leader of her Conservative party in Scotland, said any such vote could not take place until people had seen "how our new relationship with the European Union is working."

May's stance put her on a collision course with the Scottish government but left open the possibility of an independence referendum at some point after Britain has left the EU.

"My message is very clear: now is not the time," May told broadcaster ITV. "I think we should be working to get the right deal for Scotland and the U.K. with our future partnership with the European Union."

In the last referendum on independence in 2014, Scotland opted to remain within the U.K., with the backing of 55 percent of voters.

Sturgeon said on Monday the Scottish Parliament would start the process next week of asking the British government to grant permission for another independence referendum. But May's comments made clear in advance that such a request would not be acted upon.

May's decision represents a gamble. It could play into the hands of Sturgeon's pro-independence Scottish National Party, which argues the U.K. government does not respect Scotland. However, opinion polls have indicated that most people in Scotland do not want a second referendum.

Sturgeon took to Twitter to respond to May's comments, saying she did not want a referendum now but "when the terms of Brexit [are] clear and before it is too late to choose an alternative path."

In order to have a referendum at that point, Sturgeon said, preparations had to begin now. Any refusal to pave the way for a referendum would be "undemocratic" and "also proof positive that the Tories fear the verdict of the Scottish people."

Sturgeon argues she has a mandate for a new referendum as her government had said in its last manifesto that it would propose one if Britain voted to leave the EU.