The Scottish Parliament's backing for a second independence referendum must be respected, Scotland's First Minister has said.
MSPs voted by 69 to 59 in favour of seeking permission for a ballot to take place between autumn 2018 and spring 2019.
Nicola Sturgeon said her mandate for another vote is now "beyond question", and warned it would be "democratically indefensible and utterly unsustainable" to attempt to stand in the way.
Scottish Secretary David Mundell said the UK Government would decline the request.
The First Minister confirmed she will make a formal approach for a section 30 order - the mechanism for the powers to hold a referendum - in the next few days, and will set out her next steps to Holyrood after Easter recess if she is rebuffed.
The minority Scottish Government won the vote thanks to support from the Scottish Greens, and following an extended debate which was delayed by a week due to the Westminster terror attack.
The vote followed a meeting between Ms Sturgeon and the Prime Minister in Glasgow on Monday, and came the day before the process for leaving the European Union will be formally triggered.
The First Minister said: "It is now the will of Scotland's democratically-elected national Parliament that discussions should begin with the UK Government to enable an independence referendum to be held.
"Today's vote must now be respected. The mandate for a referendum is beyond question, and it would be democratically indefensible - and utterly unsustainable - to attempt to stand in the way of it.
"The Prime Minister says that now is not the time for a referendum. I agree with that, which is why I have indicated a timescale no earlier than 18 months from now, when the terms of Brexit are clear - something the PM has now indicated she agrees with.
"It is up to the UK Government to now make clear when they consider a referendum would be appropriate."
Mr Mundell told BBC Scotland: "We're not entering into negotiations on whether there should be another independence referendum during the Brexit process.
"We don't have a crystal ball as to how long that process will take. We don't recognise, for example, 18 months as being a key point in the journey.
"It will be a journey that will involve the negotiations with the EU, it may be a journey that involves transitional measures, it may be a journey that will involve significant implementation time."
A UK Government spokeswoman said: "The Prime Minister has been clear that now is not the time for an independence referendum, and we will not be entering into negotiations on the Scottish Government's proposal.
"At this point, all our focus should be on our negotiations with the European Union, making sure we get the right deal for the whole of the UK.
"It would be unfair to the people of Scotland to ask them to make a crucial decision without the necessary information about our future relationship with Europe, or what an independent Scotland would look like."
The Scottish Conservatives, Labour and the Liberal Democrats voted against another referendum.
Tory leader Ruth Davidson said: "We have made it clear: now is not the time to go back to another divisive referendum. Not when there is no public support for one. Not when the SNP said the last referendum would be once in a generation. Not when we have no clear picture as to what either Brexit or independence will look like.
"We will continue to oppose a second referendum every step of the way. The majority of people in Scotland do not want it and the SNP does not have a clear mandate to pursue it."
Labour leader Kezia Dugdale said: "There absolutely should not be another independence referendum until after Brexit. We have no idea what Brexit looks like, or how it will impact our economy and families in Scotland.
"If there is to be another vote, the people of Scotland deserve clarity on what they are being asked to vote on.
"This process cannot be a stitch-up between Nicola Sturgeon and Theresa May."
Lib Dem leader Willie Rennie said: "Scottish Liberal Democrats stood on a manifesto to oppose a divisive referendum and we will continue to do that."
Ross Greer, the Scottish Greens' external affairs spokesman, said: "It should be our responsibility, as those elected by the people of Scotland, to fight for their right to choose their own future."
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