“Scotland cannot be kept forever in an arrangement if her people wish to end it,” the former Conservative prime minister has written.
“Boris Johnson probably has a legal right to refuse to sanction a second independence referendum. But he should be wary how he uses that power.
“It is unwise to dismiss Scottish ambitions, or to delay any vote, without action to expose the reality of separation – and remedy shortcomings in the UK’s devolution settlement.
“A blunt refusal would be a still greater error if accompanied by the provocative assertion that Scots should wait another generation before voting again. Such a hardline approach is more likely to provoke a break-up than prevent it.”
The warning follows Mr Johnson’s branding of so-called “Indyref2” as “completely irrelevant” – when he suggested a 40-year gap between referendums.
In a bitter civil war, the former first minister has now set up his own party to win a “super majority” for independence – but at the risk of depriving his old party of securing that by itself.
In the article for the Financial Times, Sir John joins Gordon Brown in calling for the UK government to order an “independent assessment of the pros and cons of separation”.
“If the two governments will not commission such studies, then their parliaments should – and academia, too,” the former Tory leader said.
“In parallel, the UK must address any constitutional amendments thought to be necessary by changed circumstances, frictions in the devolution settlements or the impact of Brexit.”
Sir John describes Scotland as a “proud nation perfectly capable of self-government” – but argues most Scots can be convinced to reject independence “with facts and reason”.
“The economic cost of separation would be harsh on Scotland. There is no longer an oil bonanza to boost the economy. The fiscal deficit is above 7 per cent of economic output,” the article said.
“Over 60 per cent of Scottish trade goes to the rest of the UK, three times her trade with the EU. Does Scotland really want a trade border with England?
“The ‘Barnett Formula’ boosts Scottish public spending by nearly £2,000 per person per year from Westminster. If lost, can Scottish taxpayers make up that sum? This is not Project Fear. It is reality.”
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