Around one tenth of Scottish jobs are linked to trade with the UK, demonstrating that a "borderless UK" is a key part of Scotland's economic success, the Treasury has said.
The UK Government department has published new analysis on jobs based on research by Professor Brian Ashcroft of Strathclyde Business School.
It says more than 100,000 of these 270,000 jobs are done by women, while more than one in three are done by under-35s. Most of these jobs - 180,000 - are in the services sector, including 43,000 in financial services, the analysis states.
Meanwhile, total Scottish exports are worth £72 billion a year, with £51 billion of goods exported to the rest of the UK.
"The economic evidence shows that a borderless UK is a key part of Scotland's economic success," the Treasury says.
"The analysis demonstrates how important the UK's fully integrated single market is for the security of jobs and livelihoods in Scotland."
Scottish Liberal Democrat leader Willie Rennie said the jobs would be endangered in the event of a Yes vote.
Speaking at a Better Together event in Glenrothes, Fife, he said: "These jobs wouldn't go on day one of independence, but they would be in danger even before that.
"The sheer numbers show how connected we are as a family of nations and how much we support each other to grow.
"The combination of the ability of people in Scotland and the opportunity that the United Kingdom presents means we can record such progress then hope and believe we can achieve more.
"I'm not going to put over 250,000 jobs at risk; bring investment in renewables shuddering to a halt; see our universities' research stall. If (First Minister) Alex Salmond thinks I'm going to lie down whilst he puts all of these things on the line then he can think again."
He added: "Our ambition is to build on the 270,000 jobs that depend on trade with the rest of the UK, not risk that number by putting burdens and barriers in front of companies that want to work across the border."
A Scottish Government spokeswoman said: "An independent Scotland will still be the rest of the UK's main trading partner - and the biggest threat to Scotland's economy is from Westminster's proposed in/out referendum on EU membership, which risks taking Scotland out of a single market of more than half a billion people.
"We believe the economic gains of independence and the reindustrialisation of the Scottish economy will deliver trade, investment and supply chain benefits for both Scotland and the rest of the UK, and help to further increase Scottish exports."
Paul Fletcher, of the pro-independence Business for Scotland group, said the UK Government was using "outdated research" in the report.
Mr Fletcher, a former adviser to the UK Government, claimed: "That research on the so-called border effect is 21 years old and comes from before the North American Free Trade Area started. The research isn't even relevant to the US and Canada any more, never mind Scotland's future.
"What worries Scottish businesses is the threat to our trade that anti-EU politics brings. With both (Prime Minister David) Cameron and (Labour leader Ed) Miliband discussing the Ukip agenda about leaving the EU, there's a real danger that Scotland will be separated from a single market of 500 million people and the pool of talent that goes with it. It's the 'Westminster effect' that's causing concern, not the 'border effect'."
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