Nigel Farage will hit out at Nicola Sturgeon during tonight's televised leaders' debate, as the Ukip leader and others attempt to capitalise on fears south of the border about the SNP in the run up to the General Election.

 

The eurosceptic politician will criticise the First Minister for being "pro mass migration, pro European Union", party sources said.

In a bid to increase Ukip support among Scottish viewers he will also accuse the nationalist of being an 'establishment' figure like David Cameron.

The SNP accused Mr Farage of planning to play the contest "for laughs".

The debate takes place against a backdrop of growing concern in England about the sway the SNP could hold after May.

Yesterday, one of the world's largest fund managers, BlackRock, warned a Labour government propped up by the SNP "could turn the UK into a sort of pre-crisis Spain, with mounting piles of regional debt to fund local spending."

A new poll also showed that Londoners overwhelmingly oppose the prospect of the SNP holding the balance of power at Westminster.

That followed an exclusive poll for the Herald by TNS which showed that nearly half the UK electorate is fearful of the potential influence of the SNP.

On the eve of the debate Labour was forced on the back foot after 100 business leaders published an open letter warning that a change away from the Tory-led coalition's economic strategy would "threaten jobs and deter business".

The party later released its own letter of support signed by business leaders, cafe workers and celebrities.

But Labour also became embroiled in a tax row after shadow chancellor Ed Balls repeatedly refused to rule out drawing more middle-income earners into the higher 40p income tax band.

Mr Cameron and Mr Miliband are also expected to attack Ms Sturgeon, who will hope to find support from what she has dubbed the 'progressive alliance;' including Plaid Cymru and the Greens as she criticises the Tory and Labour leaders.

For his part, Mr Miliband has said he is preparing by getting out and talking to voters again in the wake of last week's televised showdown with Mr Cameron, which some polls declared him the victor.

Others appear less keen on the seven-leader debate. Liberal Democrat leader Nick Clegg has already tried to downplay any expectations, saying he does not expect a repeat of the 'Cleggmania' that followed the 2010 debates.

Senior Tories have urged Mr Cameron to try to rise above the fray as many of the party leaders join together to attack his government's record.

But there is also expected to be disagreement among the smaller parties as they vie for the votes of disaffected former Labour and Lib Dem supporters.

Ukip insiders said that Mr Farage, who is said to be preparing "rigorously", planned to turn his guns on the First Minister during the showdown.

A Ukip source said: "Essentially just like the rest. She is pro-EU, pro-mass migration.

"She may be Scottish, but she is just as much an establishment figure as the rest of them. She's just Scottish establishment."

A senior SNP source accused the Ukip leader of having a laugh.

He said: "Nigel Farage, ex-City trader, must be planning to play it for laughs if this is his pitch.

"The contrast will be between four men in suits who all support a cuts agenda, and three vibrant women leaders (from the SNP, Plaid, and the Greens) committed to building progressive politics - including an end to austerity - to benefit people right across the UK."

Ms Sturgeon is also expected to hit out at Mr Farage during the event, following recent accusations of racism against Ukip's Scottish MEP.

The debate could prove an endurance test with the party leaders given just one break during the two-hour event, due to be held in Salford, prompting predictions that none will drink the water provided on stage.

Ahead of the debate, Scottish Labour leader Jim Murphy said while Scotland could decide the outcome of the election, but of the seven people at the podium only Mr Miliband or Mr Cameron could become Prime Minister.

But Professor Jane Green, from the University of Manchester, said that the odds were stacked against Labour getting extra votes in Scotland as a result of the debate.

"Vote switching, which happens beneath the surface, could easily throw up unexpected results during the campaign. But while Scottish Labour may convince its previous voters to come back to the fold from the SNP, we do know that the odds are certainly stacked against them," she said.

"We find 86 per cent of Labour voters who had switched since 2010 say they will now stick with the SNP."

Professor Ed Fieldhouse, also from the University of Manchester, said that while across the UK Labour and the Tories enjoy the loyalty of around three quarters of their 2010 voters "Liberal Democrat loyalty is an astonishingly low - 23 per cent.

"This is almost unprecedented for a supposedly major party, and has reduced the Liberal Democrats to a level of support not seen since their formation.

"Indeed if they do not improve on their current 7.5 per cent of the vote before the General Election it would represent their lowest vote share in a general election since 1970 when they won only 6 seats."

Green Party leader Natalie Bennett will speak first in the opening statements of the debate while Mr Cameron will speak last.

Nick Clegg for the Liberal Democrats, Ukip's Nigel Farage, Plaid Cymru's Leanne Wood and Scottish National Party leader Nicola Sturgeon are also taking part.

ITV said the two-hour live debate, to be screened from 8pm to 10pm, will allow each leader to give an uninterrupted one-minute answer to each question posed by a studio audience of around 200 people.

There will be up to 18 minutes of debate on each question with "four substantial election questions" addressed.

Leaders will not see the questions in advance and an "experienced editorial panel" will select them.

Northern Ireland's DUP has also reiterated its complaint that it is being discriminated against by being left out of the event.

Ms Sturgeon said: "This election is unprecedented in modern times and is a unique opportunity for voters in all parts of the UK.

"And tonight's historic debate signals that it is not just Scotland that wants change, but people across the whole of the UK - it shows the mould of two and three-party politics at Westminster has been broken."