Theresa May still does not have a coherent Brexit strategy four months after the vote to quit the EU, Nicola Sturgeon has warned.
Scotland's First Minister said a Brexit summit in Downing Street between the PM and devolved leaders had contained no new information, and left the Government's stance "no clearer".
"I don't know any more now about the UK Government's approach to the EU negotiations than I did before I went into the meeting," Ms Sturgeon said.
The two-hour long talks, which Ms Sturgeon branded "feisty", saw Mrs May and the First Minister clash during a "very frank exchange of views."
Ms Sturgeon rounded on a warning from Number 10 that the devolved administrations must not try to undermine the UK's negotiating position as "nonsense" as she insisted London did not actually have a firm grasp of what it wanted.
Ms Sturgeon dismissed talk of undermining Britain's EU withdrawal deal, saying: "I think that is nonsense, and it is not what anybody is seeking to do.
"To be brutally frank about it, you can't undermine something that doesn't exist, and from everything I have heard today in Downing Street there isn't yet a UK Government negotiating position.
"I've no interest in undermining that when it does exist, but I do have a massive interest in protecting Scotland's interest. What I'm not prepared to do is stand back and watch Scotland driven off a hard Brexit cliff-edge."
Ms Sturgeon said the tone of the meeting had been robust.
"We had a very frank exchange of views. I don't mind admitting large parts of the meeting were deeply frustrating," she said.
Ms Sturgeon agreed when asked by a journalist if the Government had provided such little information because it "didn't have a clue".
Number 10 said Mrs May had told the devolved administrations she would strike a bespoke Brexit deal that works for the whole of the UK, and wanted their input in shaping a deal.
Mrs May said: "The great union between us has been the cornerstone of our prosperity in the past - and it is absolutely vital to our success in the future.
"The country is facing a negotiation of tremendous importance and it is imperative that the devolved administrations play their part in making it work.
"We have important work to do for the UK in terms of negotiating a smooth exit from the EU and getting the best possible deal for the whole of the UK."
Ms Sturgeon was dismissive about a so-called hotline being set up between the devolved capitals and Brexit Minister David Davis.
"Having a direct line is good, but it won't mean much if David Davis is not prepared to say anything when he picks up the phone. I don't have any interest in a lot of silent calls."
The First Minister said Mrs May should be under no illusion about her party's determination to hold a second independence referendum if the SNP felt that was in the best interest of Scotland.
Ms Sturgeon said it would be inconceivable for London to try to block such a vote.
"It would be inconceivable for the UK Government to be in that position given that it's them that has put us into this position in the first place."
Scotland will bring forward specific proposals for a so-called flexible Brexit that would keep the nation in the single market, even if the rest of the UK was not part of the trading agreement, in the next few weeks, Ms Sturgeon said.
Labour's Carwyn Jones said there had been no details about what happens next in the divorce talks, and warned "time is not on our side".
The Welsh First Minister said access to the single market was the most important issue, but dismissed suggestions by Nicola Sturgeon that Scotland could remain part of the trading agreement even if the rest of the UK did not.
"We need to make sure we continue to sell in one of the world's biggest markets on the same terms as we do now," he told Sky News.
Martin McGuinness, Northern Ireland Deputy First Minister, said there was a "joint responsibility" to manage the challenges posed by Brexit.
He told reporters: "As this process moves along, we need to be at the heart of it," as he warned against the imposition of a "hard border" with the Republic.
Downing Street earlier said leaders of the devolved administrations must not undermine the negotiations.
"We have been very clear that we should be working together to secure the best possible deal for the whole country," the Prime Minister's official spokeswoman said.
"We expect representatives of the devolved administrations to act in that way and to in no way undermine the UK's position."
Northern Ireland Secretary James Brokenshire reiterated the Government's commitment to maintaining current border arrangements with the Republic of Ireland.
Downing Street played down suggestions that the Prime Minister's preferred negotiating strategy was to pursue a free trade agreement with the EU after she appeared to suggest that was the plan during exchanges in the Commons.
After Mrs May updated MPs on last week's summit in Brussels with EU leaders, Tory Sir Edward Leigh asked: "Is not our objective this; that we want to conclude, having adopted every last EU law to our laws on Brexit day, we want to conclude a free trade agreement which is overwhelmingly in the interests of the rest of Europe?
"And incidentally, this would do so much for the poorest nations in the world, as we lead the battle in the world for a free trade and prosperous world?"
Mrs May replied: "I agree."
Asked about the comments, the Prime Minister's official spokeswoman said: "The point I took from it was that she was agreeing it was in Europe's interest to work with us to reach a deal."
Pressed on whether the PM's preferred negotiating position was a free trade agreement with the EU, she replied: "The PM has been very clear. We are going for a bespoke arrangement."
Mrs May told MPs there will be a "series of general debates" on the UK's future relationship with the EU before and after Christmas.