The vote for a second Scottish independence referendum must be respected, Nicola Sturgeon has said.
MSPs voted by 69 to 59 on Tuesday in favour of seeking permission for a ballot to take place between autumn 2018 and spring 2019.
Scotland's First Minister 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.
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.
''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 said there would be no negotiations about an 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," the Scottish Secretary said.
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."