THE First Minister is expected to challenge fears whipped up by David Cameron that the SNP would be a negative influence at Westminster in a major speech in London today.

Nicola Sturgeon is due to tackle head on the Prime Minister’s fierce and ongoing criticism of the nationalists and instead tell voters that the SNP will be a constructive force at Westminster for the whole of the UK after the General Election. She will say her party, which could hold the balance of power after May 7, can work to help every part of the country and will pledge not only to serve Scotland’s interests.

Ahead of the Chancellor’s budget statement on Wednesday she is expected to set out her approach to budget setting, highlighting how tax and spending decisions could be taken differently in relation to areas such as oil and gas, the work allowance and Trident.

The lecture at the London School of Economics follows the First Minister’s speech in the capital last month, in which she called for £180 billion of extra spending on public services by 2020.

Today she is expected to underline a commitment to reducing the deficit and improving economic growth in ways which do not harm the most deprived members of he community.

“We will argue for a moderate approach to deficit reduction – one which doesn’t penalise the vulnerable and harm economic growth,” she is expected to say.

“And we will argue for a different way of doing things. Budgets should take equality impacts seriously and use consultation more effectively.

“If we can be a constructive voice in the months and years ahead, we won’t just serve Scotland’s interests; we’ll help where we can to bring positive change across the UK as well.”

The First Minister is expected to say the SNP would pursue its priorities with a “positive spirit”.

She will say: “We were constantly told by the UK Government before the referendum that Scotland was an equal and valued member of the United Kingdom.

“So don’t be at all surprised if SNP MPs and the SNP Government now start taking them at their word.

“We have clear and constructive views on many aspects of UK policy which affect Scotland deeply, views which we know are often shared by many people elsewhere in the UK.”

Polls have suggested that the SNP may take as many as 50 of Scotland’s 56 seats at the General Election, prompting an increasing chance of a hung parliament as Labour’s support crumbles north of the Border.

Earlier this month the SNP Westminster leader said there is “an offer on the table” to help Ed Miliband into Downing Street if Labour became the biggest party after the General Election but fails to win a majority.

Angus Robertson, MP for

Moray, urged Labour to realise it would have to work with the Scottish National Party if it fell short of outright victory. It came after David Cameron called

Miliband “despicable” for trying to “crawl into Downing Street” with support from the SNP. The Prime Minister repeatedly criticised the Labour leader for attempting to win office “on Alex Salmond’s coat-tails” during exchanges in the Commons.

Last week the Conservatives unveiled an election poster showing a miniature Labour leader in the top pocket of a giant Alex Salmond. Cameron has also called a Labour-SNP coalition “a nightmare” scenario.