Gordon Brown has attacked the Conservative Government for stoking a "dangerous and insidious" English nationalism that he said is pushing England and Scotland apart.
The former prime minister told an audience at the Edinburgh International Book Festival that the future of the Union would be determined in the next 12 months by events in England rather than in Scotland.
Mr Brown said that post-referendum Scotland was at a turning point facing two futures, a choice between a 19th century model of "absolute sovereignty" and a 21st model of "shared sovereignty" that recognised the interdependence of the world economy.
But he warned that nationalism in England was a factor that Scotland had no control over and one that had "changed dramatically" over the last year.
He laid the blame at the door of the Conservative Government which he said had been willing to "turn on the tap" of English nationalism.
Mr Brown highlighted David Cameron's decision to push for English votes for English laws in the wake of last year's no vote in the referendum on Scottish independence.
The Tory proposals would create two classes of MP at Westminster and would be unsustainable in the long run, he said.
Mr Brown also criticised the UK Government's decision not to devolve the power to top up welfare benefits to the Scottish Parliament as recommended by the post-referendum Smith Commission on further devolution for Scotland.
Failure to do so would leave the Scottish Parliament powerless to mitigate impending welfare cuts, he said.
Repeating calls for a constitutional convention for the UK, Mr Brown said: "The third thing that I think is dangerous and almost insidious is what happened in the last few months, when clearly the tap of English nationalism was turned on by the Conservatives.
"The nationalists in Scotland were playing the Scottish national card, there's no doubt about it, but instead of playing the British unity card the Conservatives decided to play at the general election the English nationalism card.
"All this was designed to give the idea that there was a Scottish menace, a Scottish danger, a Scottish risk.
"The tragedy of all these three changes that have been brought about in the last year by decisions of the Government of the day is that they are pushing Scotland and England further apart.
"So even if the Scottish people decide that they were happy to have a balanced relationship with the UK where there was autonomy balanced by sharing, there is this new dimension in the arrangement - which will come alive again I'm afraid in the European referendum - and this I'm afraid is driving us apart.
"In the next 12 months I believe the fate of the union will be determined not so much by what we're saying here in Scotland but by many of these developments in England which I think are dangerous and in some cases insidious."
He added: "I don't think this is an inevitable process. I do think that the damage that was done in the last few months may be irreversible, I hope it isn't.
"That's what I would urge today, that we find a common ground because in the long run we are in an interdependent world, we've got to find a way of working together."
An SNP spokesman said: "This speech shows why people are moving away from Labour in Scotland.
"After working arm-in-arm with the Tories for two-and-a-half years in the No campaign, Labour were adamant that powers over issues like welfare should be retained at Westminster.
"Now they are blurting out the truth as the Tories step up their damaging cuts. Gordon Brown should apologise for Labour so shamelessly misleading people in Scotland during the referendum."