Pursuing the Brexit Bill without the consent of the UK’s three devolved legislatures was necessary, No 10 suggested, because Britain’s withdrawal from the EU was a “very exceptional” development.

Downing St’s justification came as Boris Johnson said he was “1,000 per cent” committed to the Union of the United Kingdom and made clear indyref2 would not happen for at least 15 years “if ever”.

Speaking from Downing St in The People’s PMQs broadcast on Facebook, Mr Johnson declared: “We commit 100, 1,000 per cent to the Union, the most successful political partnership in the world; we’re going to defend it, we’re going to protect it.”

The PM decried the SNP Government for “obsessing about breaking up our Union” rather than concentrating on the people’ priorities of better healthcare and better education.

“Look,” declared Mr Johnson, “Scottish education is legendary. It produced Michael Gove, the Chancellor of the Duchy of Lancaster. Andrew Neil is a product of the Scottish education system; picking out something entirely at random. It’s a fantastic system.

“But recently, Scotland, through no fault of the pupils, has been slipping down the PISA rankings[in] both maths and science. It’s time the SNP really focused on the priorities of the people of Scotland on education, on healthcare, on cutting crime or cutting drug addiction rather than breaking up the Union.”

The PM argued that people had no clear idea of what Scottish independence would mean. Would there be a border at Berwick? What would an independent Scotland’s currency be called?

Insisting the line, that the 2014 referendum was a once-in-a-generation event, should be stuck to, he added: “Whatever your view of human biology, five years is not a generation nor is 10 years...nor 15. There’s a long time before it will be necessary, if ever, to put that question back to the people.”

Earlier, Mr Johnson’s spokesman was asked if the Sewel Convention, which states the UK Parliament "will not normally" legislate for devolved matters without the consent of the devolved legislatures affected, was now dead in the water given the UK Government had pressed on with the Brexit Bill in the face of opposition from the Scottish Parliament and the Welsh and Northern Irish assemblies.

“I would suggest the circumstances of our departure are clearly very exceptional. We had a referendum that was voted on. We respect referendums," he explained.

During PMQs, Ian Blackford for the SNP claimed the Tory Government had launched an “unprecedented attack” on devolution and called for the Brexit Bill to be stopped because of its rejection by the three devolved legislatures.

He told Mr Johnson: “All three parliaments and even the House of Lords have called on you to end your Government’s attack on devolution. Will you stop the attack on our parliaments?”

But the PM, repeating the once in a generation line, talked up the cross-border trade and investment in Scotland, telling Mr Blackford to Conservative cheers: “We support manufacturing in Scotland; they support nothing except manufacturing grievances and they know it.”