Unionists must present options to maintain Scotland's relationship with the European Union or risk the break up of the UK, a think tank has warned.
The Institute for Public Policy Research (IPPR) said the UK Government and the Scottish Government must explore these options with "equal vigour".
First Minister Nicola Sturgeon has pledged to "explore all options" to keep Scotland in the EU, but said a second independence referendum is "highly likely".
British Prime Minister Theresa May has said she is "willing to listen to options" that the SNP present - but she has described some of the early ideas as "impracticable" and Scottish Secretary David Mundell said the idea of Scotland remaining in the UK and EU is "fanciful".
IPPR director Russell Gunson said the Scottish Government lacks "a genuine commitment to the UK" so it is up to unionists to present options for Scotland's EU future if they want to protect the UK.
Writing in the Scotland on Sunday, he said: "It is those across the UK who do not wish to see Scottish independence, or those open to exploring all the options, who will need to come forward with other options."
These options "need to be explored with equal vigour and rigour by both the UK and Scottish Governments", he said.
"The latter is certainly giving the impression of being up for exploring all the options, but without a genuine commitment in the rest of the UK, and without significant reform to Britain as a whole, a tailored approach to Brexit will not be deliverable.
"And with that, Scotland's options would be narrowed significantly."
He added: "One of the only things we can be sure of at this stage is that the UK - including Scotland or not - will need to change fundamentally, or it will be forced to, in order to meet the multiple mandates delivered by the EU referendum result."
SNP Westminister leader Angus Robertson said on Friday: "Scotland is on the brink of independence."
He followed up on Sunday by outlining a series of issues in which "Scotland has been ignored, outvoted or overruled" since the general election.
He cited the election of the Conservatives, the austerity budget, Brexit, Trident, bombing Syria, refugees, the Trade Union Bill, scrapping the Human Rights Act, and blocked reforms of the House of Lords and votes for 16-year-olds.
He said: "If the UK government and Westminster parties continue to act against Scotland's wishes as they are - they shouldn't be surprised when the Scottish people conclude independence is the only viable option."
However, oil tycoon and former Scottish Government jobs commissioner Sir Ian Wood has warned Scotland "would have vern little influence in Europe" as a small independent member of the EU.
He said EU accession is "a long process and one that would be damaging, with a lot of uncertainty for the oil and gas industry"
He told the Sunday Times Scotland: "I was in favour of remaining in Europe. I have this idea we are becoming too divisive in the world.
"Everyone wants to be independent, but I think bigger economic units are more successful. We actually have to be better at living and working together."