THE crucial referendum battle to win over Labour supporters will intensify today when Ed Miliband delivers a speech claiming a No vote is the only way to achieve social justice.

The Labour leader, due to speak in Lanarkshire, will insist he is on course to become Prime Minister in next May's General Election and will set out his plans to build a "fairer" country.

His latest campaign visit comes a day after Alex Salmond and Nicola Sturgeon launched an audacious bid to win support for independence from a majority of Labour voters.

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Speaking during a visit to Bathgate, West Lothian, yesterday, Ms Sturgeon insisted Labour supporters were "moving in significant numbers to Yes".

Traditional Labour heartlands have emerged as a potentially decisive referendum battleground following a poll this week that showed backing for Yes among the party's supporters has risen from 18 per cent to 30 per cent in the space of a month.

In a sign of growing nervousness among Unionists after the No campaign's lead shrank to just six percentage points, Tory backbencher Sir Edward Leigh used Prime Minister's Questions to tell David Cameron and the other main party leaders they were "complacent" and were facing a "national humiliation of catastrophic proportions" if Scotland voted to leave the UK.

Meanwhile Goldman Sachs, the US-based bank, warned an independent Scotland would face "painful" cuts to public spending as a result of higher borrowing costs.

It also warned of a possible run on Scottish banks, with savers shifting their money out of the country, ­regardless of whether Mr Salmond's proposals to share the pound in a currency union with the rest of the UK could be agreed.

Leading stockbroking firm Hargreaves Landsdown said yesterday it was receiving an increasing number of calls from concerned clients seeking reassurances that a Yes vote would not affect their holdings.

Mr Miliband has switched his planned visit from Aberdeen to Blantyre to spearhead the effort to shore up No support in Labour heartlands. He will join activists canvassing door-to-door today. Gordon Brown and John Reid are also set to step up campaigning in the coming days.

In his planned speech, the Labour leader will highlight pledges to freeze energy bills, raise the minimum wage, introduce a 50p higher rate of income tax and a bankers' bonus tax if he becomes Prime Minister.

He will also repeat Labour's promise to devolve more powers over tax, social security and schemes to support the unemployed.

Mr Miliband will contrast Labour's programme with the SNP's proposal to cut corporation tax, the main tax on business profits, by 3p if Scotland becomes independent.

He is expected to say: "I want to be very clear about the change I offer you in just eight months, as Prime Minister."

Describing the Conservatives as "defecting, divided and downhearted", he will also seize on Scots Tory leader Ruth Davidson's admission, in an STV debate on Tuesday night, that she did not believe David Cameron would win the election.

"The Tories are on their way out," he will say. "Even Ruth Davidson says David Cameron isn't on course to be in Downing Street in a matter of months. Electing a Labour government is the way to change Scotland. It is the way to build a just Scotland. The choice for social justice is No, not Yes."

Party sources said Labour was engaged in its "biggest ever ground campaign," speaking to 32,000 Scots face-to-face on their doorsteps in the past week.

In a rival pitch to Labour supporters yesterday, Ms Sturgeon repeated Yes campaign claims - strenuously rejected by the pro-UK side - that privatisation of health services south of the Border would damage the NHS in Scotland.

She said: "Labour created the health service and with a Yes vote in the referendum Labour voters have a once-in-a-lifetime opportunity to protect our public NHS. Labour supporters believe in social justice.

"They want to protect public services, they want to ensure that we've got a social security system that protects the most vulnerable and the best way to get these things is to put Scotland in control of our own resources and the decision that shape the kind of country we are."

It is understood that Better Together, the cross-party pro-UK campaign headed by Alistair Darling, is concerned support is ebbing away in a number of former Labour strongholds including Coatbridge, Motherwell, Inverclyde and parts of Ayrshire.

Bookmaker Ladbrokes yesterday cut the odds on Scotland becoming independent to 5/2, in from 5/1 a week ago. The odds for a No vote have drifted from 1/8 to 2/7 over the same period.