The argument the prime minister uses against a second referendum – that it was a “once in a generation” vote – no longer holds water because of the Brexit vote, it warns.
The SNP leapt on the memo, prepared for Michael Gove and other ministers, as proof that the government is “in panic mode”, as polls show a consistent and growing lead for the ‘Yes’ campaign.
It was revealed, by Bloomberg News, as a leading pollster said women were now swinging behind independence, wiping out the “gender gap” that had previously shown greater support among men.
Mr Johnson is expected to refuse so-called ‘Indyref2’, even if – as seems certain – the SNP triumphs in next May’s elections to the Holyrood parliament and claims a mandate.
But the 21-page memo admits that “Brexit has changed the game”, after two-thirds of Scots ended up with the decision to leave the EU they had opposed.
“If the SNP builds on this momentum then the endpoint could be a full-blown constitutional crisis or a second independence referendum,” it states, Bloomberg reported.
“Either of these outcomes would consume significant political capital for the government.”
It then suggests turning to Brussels to dash hopes of reversing Brexit, by “co-opting the EU into demonstrating that there is no viable pathway to renewed membership.”
But it also admits: “Put simply, there are not enough Leave voters to convert to the ‘No’ side to make up for the movement of Remain voters into the ‘Yes’ camp”.
The memo suggests three routes to mitigate the pressure, the report said – “new accommodation, new constitutional settlement, and cooperation rather than confrontation.”
It describes the first step as a “velvet no”, that rejects a second referendum in the short term and buys time.
The government should then focus on a ‘Four Nations, One Country’ policy by transferring further financial powers and exploiting issues such as immigration.
Kirsten Oswald, the SNP’s deputy leader at Westminster, said: “This leaked memo reveals that the Tories are in panic mode because people in Scotland know Boris Johnson’s government can’t be trusted to act in Scotland’s interests.”
Professor Sir John Curtice said studies from 2014 found men were much more likely to vote Yes than women, but recent polls found this gap has shrunk or vanished altogether.
At the same time, a number of pollsters have found support for independence has risen above 50 per cent – and to as high as 58 per cent.
“What we can say on the basis of the two polls is that there continues to be a sustained lead for Yes – it is now nine polls in a row that have put Yes ahead, a wholly unprecedented picture,” the elections guru said.
“These same nine polls on average put the figure for both men and women at 54 per cent too.”