Nicola Sturgeon has said the British Prime Minister has no rational argument against a second independence referendum following discussions between the two leaders over the Brexit timetable.
The First Minister wants to hold another vote on leaving the UK between autumn 2018 and spring 2019 - a timescale Theresa May is set to reject.
Mrs May has said a referendum during that period would be "unfair" to voters because they would not have all the necessary information to make a choice.
Following a meeting in Glasgow, Ms Sturgeon insisted the Prime Minister had been clear the terms of the UK's divorce from the EU and the details of a new free trade deal would be known within two years.
"I think it makes it very difficult for the Prime Minister to maintain a rational opposition to a referendum in the timescale I have set out," Ms Sturgeon said.
"I think she has got a perfectly rational opposition to a referendum now, which is why I am not proposing it.
"But I think based on the discussion today I would struggle to see what her rational opposition to it would be in the timescale we have been talking about."
Ms Sturgeon added: "She (Mrs May) is absolutely adamant that she believes the terms of Brexit, by which she means the exit terms, the divorce deal, and the detail of the comprehensive free trade agreement - in other words the future relationship between the UK and the EU - will be clear before the UK exits the EU.
"When I put it to her that what she was suggesting was that in a period of 18 months to two years from now, the terms of the future relationship of the UK and the EU would be clear, she said yes that is what she was saying."
The meeting came just a day before the Scottish Parliament is expected to pass a vote in favour of seeking another independence referendum, and two days before Mrs May is due to trigger Article 50.
Ms Sturgeon described the talks as "cordial" and "business-like", although she said Mrs May had made no offer on powers to be devolved to Scotland as part of the Brexit process.
"I think it's fair to say that there is still no real guarantee that powers repatriated to Brussels in areas that are currently devolved will not end up being centralised at Westminster, and there was no real willingness to talk about powers beyond that, for example powers over employment law or immigration."
Asked about what would happen if her call for another referendum was formally rejected, Ms Sturgeon said: "I will set that out in due course. I actually have views in my mind around that.
"If their position remains as it is right now, I will set out to Parliament what I think the next steps should be."
Ahead of the meeting, Mrs May told reporters her position will not change on Ms Sturgeon's call for a referendum by spring 2019.
She would not be drawn on whether a vote could take place further into the future, restating her view that ''now is not the time'' for another ballot.
The Prime Minister also vowed to build a ''more united nation'' as Britain leaves the EU.
Speaking after a meeting with officers from Police Scotland in Govan, the Prime Minister said: ''My position is very simple and it hasn't changed.
"It is that now is not the time to be talking about a second independence referendum and that's for a couple of reasons.
''First of all, now is the point when we are triggering Article 50, we're starting negotiations for leaving the European Union. Now is the time when we should be pulling together, not hanging apart. Pulling together to make sure we get the best possible deal for the whole of the UK.
''Also I think it would be unfair on the people of Scotland to ask them to make a significant decision until all the facts were known, at a point where nobody knows what the situation is going to be."
Later in an address to staff at the Department for International Development in East Kilbride, South Lanarkshire, she said: "As Britain leaves the European Union and we forge a new role for ourselves in the world, the strength and stability of our Union will become even more important."
"As we look to that future and as we face this great national moment together, I hope you will continue to play your part in the great national effort we need to build the stronger Britain, the fairer Britain, the more outward-looking Britain and the more united Britain that I am determined we should be once we emerge from this period of national change."
A Downing Street source said Mrs May updated the First Minister in a "courteous and friendly way" about the letter she will send to trigger Article 50 and start the Brexit process.
Ms Sturgeon only mentioned Section 30 - the legislation which could allow her to hold a referendum on independence - "briefly at the end" of the meeting and the Prime Minister's views are "well known on this issue", the source said.