There are growing fears that Scottish independence may not be a "velvet divorce", with an angry public mood in the rest of the UK pushing political parties into rejecting an amicable break-up.
"The consequences, not just for England, but Wales and Northern Ireland, are frankly unimaginable," one senior Whitehall official said yesterday.
Here are seven things politicians in the UK are now worrying about as polls indicated the vote on September 18 was practically too close to call:
Some MPs fear that the peace process in Northern Ireland, or at least order on the streets, could be at risk, amid renewed demands for a united Ireland.
There would inevitably be calls for a stronger Welsh Assembly, more devolution to the English regions and even an English Parliament.
Cabinet ministers have described the SNP's timetable for independence as "completely unrealistic", with complex issues such as the Trident nuclear weapons base on the table.
It is now dawning on MPs that a public backlash from English voters against the Scots could force the Government to take a much harder line than ministers expect, and force the Conservatives to take a hardline stance on the negotiations in their manifesto at next May's general election.
A Scottish breakaway would provide yet more ammunition for Ukip. If Nigel Farage demanded a tough line in the talks, the Tories might have to match it. In turn, that could put pressure on Labour to follow suit.
David Cameron has dismissed the idea that he would have to resign after presiding over the end of the 300-year Union. But then he has to say that. Cameron critics on the Tory backbenches have already started to discuss whether a Yes vote next week could be the trigger for a leadership coup.
A Yes vote could also provoke calls for next May's general election to be delayed, on the grounds that there would be little point in electing 59 Scottish MPs for 10 months until "independence day".