The "narrow-minded nationalism" of both Ukip and the SNP is the wrong direction for Britain, the Chief Secretary to the Treasury has said.
Danny Alexander said the parties led by Nigel Farage and Alex Salmond rely on the "myth of exceptionalism" - a suggestion that leaving either the European Union as Ukip wants, or exiting the UK as the SNP is campaigning for, would result in a better deal for voters.
But the UK Government minister said neither party can "say with certainty what life would be like after walking away".
While he said Ukip and the SNP are "very different parties", he used a speech in Paris to condemn both their core policies.
"Both parties' narrow-minded nationalism is - I believe - the wrong direction for our country to move into," Mr Alexander told an audience at the Dauphine University.
"And a decision for Scotland to leave the UK, or for the UK to leave the EU, would be politically irreversible. A decision that could be taken only once, but with consequences that would last down the generations."
Arguing in favour of the UK remaining in the EU, Mr Alexander said: "The suggestion is that over 10% of our jobs are linked directly to our membership of the European Union.
"So why would any party, which truly understands Britain's national interest, want to do anything that could put these jobs at risk?"
He also argued that Scotland benefits from its position in the UK, saying: "Scottish interests in defence and security have benefited enormously from its role and place in the United Kingdom."
He also insisted people north of the border enjoy "levels of public service spending that could not be matched in an independent Scotland without big tax rises".
He dismissed suggestions from Ukip and the SNP that they could maintain the benefits of close co-operation between nations.
"Ukip claim that Britain can pick and choose those parts of European and international obligations it wants to hold on to, while throwing over the rest," he said.
"The SNP claim that Scotland can keep the pound and other British institutions without sharing economic or fiscal policies.
"But neither Scotland nor Britain can dictate the terms on which they co-operate with other states, cherry-picking what they like and rejecting everything else.
"Nigel Farage wants to persuade us that the other countries of the EU will have no alternative but to accept the terms a Eurosceptic Britain would offer.
"Alex Salmond wants to persuade us that the UK will have no alternative but to accept his demands - even though support has all but disappeared for his idea that a currency union could be established between Scotland and the remaining UK.
"Both rely on the myth of exceptionalism - the idea that if Scotland leaves the UK family, or the UK leaves the European family, those left behind will go out of their way to give them a unique and better deal than any country has otherwise achieved, sacrificing their own interests in the process.
"In reality, neither the SNP nor Ukip can say with certainty what life would be like after walking away. But in order to persuade people to support them they deny the risks, uncertainty and loss that must come with going it alone."
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