Promising a referendum on Britain’s place in Europe was always a rash gamble — a tactical swerve blind to the strategic consequences. The stakes have risen. The rest of Europe does not want to see the Brits depart, but the EU would muddle on. For the UK, the choice has become existential. If Britain leaves Europe, Scotland will leave Britain. The union of the United Kingdom would not long survive Brexit.

The referendum was offered to appease troublesome eurosceptics in David Cameron’s Conservative party. Some hope. There are signs the prime minister has begun to appreciate what is at stake. Never mind talk that he may be remembered as the leader who split his own party, or as the architect of Britain’s retreat from its own continent. History will be even less kind if it records that a device to quell a Tory rebellion about Europe led to the unravelling of England’s union with Scotland.

Mr Cameron’s government has lowered its sights accordingly. When Philip Hammond toured European capitals before the May election 25 of his 27 counterparts told the British foreign secretary that they would not rewrite the basic texts of the EU to accommodate British exceptionalism. Angela Merkel, the German chancellor, is particularly insistent that the Union’s organising “acquis” is sacrosanct. So the prime minister’s pre-election promise of “full-on treaty change” has made way for a more modest set of demands.