Business Secretary Vince Cable will today suggest that Scots would have lower living standards under independence than if they stay with the UK.
In a speech in Glasgow, Mr Cable will say: "To continue job creation - to raise living standards - across the UK, we need balanced economic growth … I don't believe Scottish independence can sustain these conditions as effectively and efficiently as the UK."
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The SNP said an earlier draft of Mr Cable's speech admitted that London was an economic drain on Scotland. SNP Treasury spokesperson Stewart Hosie MP said: "This admission from Vince Cable has let the cat out of the bag."
Meanwhile, Ed Miliband has ordered all 257 Labour MPs to campaign in Scotland at least once before the independence referendum.
The Labour leader said they should make clear to Scots that people from the rest of the UK want them to stay.
He told a meeting of Labour MPs last night that he expected all of them all to make a visit north of the Border before September 18.
Earlier he had told MPs that the independence vote would be "one of the most important decisions for generations".