Mr Miliband urged voters in the Labour stronghold of Motherwell, North Lanarkshire, to reject independence and the low tax policies of SNP leader Alex Salmond - who he said is being inadvertently led towards a Conservative agenda by the "race to the bottom logic of independence".
"The SNP tell you that the way to tackle social injustice is with independence," he said.
"I submit to you that that is wrong.
"A clue to what Alex Salmond's commitments are is his first commitment, which is not on the NHS or even the bedroom tax. It's a 3p cut in corporation tax for the large companies.
"How can that be the first commitment of somebody who claims to be a social democrat, who claims to be committed to social justice?
"It's because he knows that if you separate two countries which have hundreds of years of history you draw an international border.
"That produces companies who say: 'If you don't do what we say we'll move across the border. If you don't lower your tax rates, if you don't make your wages and conditions worse, if you raise wages and conditions, we're going to cross the border.'
"It's what I call a race to the bottom in wages, conditions, tax rates and all of that.
"Is a race to the bottom going to tackle the cost of living crisis that I talked about? Is it going to tackle inequality? It is not. It's going to make it worse.
"Part of my argument against the Tory Government in London is that they believe in a race to the bottom.
"(Prime Minister) David Cameron and (Chancellor) George Osborne believe that the way we will succeed as a country is by low wages, low skills, worse terms and conditions and making it easier to fire people.
"I'm not saying to you that Alex Salmond believes in his heart like they do in the race to the bottom, but he is being pushed into that by the logic of independence.
"The logic of independence takes you to two countries competing against each other where the companies have the power and the governments are at their mercy."