NICOLA Sturgeon will tomorrow cast the SNP as a party for the entire UK with a manifesto promising her MPs would try to change the UK's welfare system and foreign policy.

The First Minister will set out plans to reverse £3bn of cuts to disability benefits and make the UK government formally recognise Palestine to advance peace in the Middle East.

The manifesto will also present the SNP's alternative to austerity cuts based on increased public spending, £180bn of extra borrowing and cancelling the renewal of Trident to save £3bn a year, with the money invested in health, education and childcare instead.

In a break with SNP tradition at Westminster - and as a lure to an alliance with Labour - Sturgeon will also commit the party to voting on English issues such as NHS investment.

Buoyed by polls suggesting the party could advance from six to 50 seats in May, the SNP claimed that after years on the sidelines it now had "complete relevance" to a Westminster election, as it could hold the balance of power and determine who is Prime Minister.

Labour said the key test of the manifesto would be what it said on the SNP's plan for Scotland to rely purely on its own taxes under full fiscal autonomy (FFA).

Scottish Labour leader Jim Murphy, who yesterday published a new analysis claiming FFA would leave Scotland £40bn worse off by 2020, said anything less than a detailed costing of the policy would be a failure of credibility.

The Sunday Herald understands the SNP manifesto will prioritise reversing the "callous" cuts planned for disability benefits by the LibDem-Tory coalition.

The SNP will demand a halt to both the replacement of Disability Living Allowance (DLA) with Personal Independence Payments (PIP), and the roll-out of Universal Credit.

The party will also push for an urgent review of how disability benefits are assessed.

Switching from DLA to PIP is expected to see 100,000 working age people in Scotland lose some or all of their benefit by 2018, an average loss of £1120 a year.

At a UK level, the cut amounts to £3bn a year, with over 1m people losing out.

Campaigning with her predecessor in Gordon yesterday, the First Minister said: "In this election Scotland has an opportunity like never before to make its voice heard at Westminster, but it is clear that the only party who can be that voice is the SNP.

"Polling shows that the SNP has become the national party of Scotland in all senses - leading in the polls across all areas and demographics of Scottish society."

Alex Salmond, the SNP candidate in Gordon, added: "Ours will be a voice for a new, better and more progressive politics at Westminster - for everyone across these islands."

The SNP will also push the next UK government to formally recognise the Palestinian State alongside Israel to help pursue a two-state solution to conflict in the Middle East.

SNP Westminster leader Angus Robertson said the SNP would try to shape "a more positive international agenda" for the UK.

He said: "Securing long-term stability in the Middle East is a top priority. We would encourage both Israel and Palestine to reach a sustainable, negotiated settlement."

The UK-wide approach is meant to sell the SNP to English voters wary of a Labour-SNP pact, although the SNP's critics claim it is about putting English voters off Ed Miliband to get a second Tory term and boost the chances of independence.

A senior SNP source said: "This will be a manifesto for delivery UK-wide, with our alternative to cuts its centre-piece. For perhaps the first time, the SNP have proved our complete relevance to a Westminster general election - not least with Nicola Sturgeon's hugely impressive performances in the TV debates.

"If we earn people's trust on May 7, we can be in a decisive position to help deliver a bolder programme than Labour on their own are willing to. Being a stronger voice for Scotland and a progressive force in UK politics are two sides of the same coin."

Murphy said the manifesto was the SNP's last chance to be honest about FFA.

He said: "The SNP's key general election policy is to cut Scotland off from UK-wide taxes, meaning an end to the UK pension and welfare state here.

"Unless the SNP come clean about the devastating impact of their plan to scrap the block grant and turn money away at the Border their manifesto will fail the credibility test."

The LibDems said FFA meant "devastating" cuts, including £1bn a year lost in Glasgow.

Warning of the "chaos of a Labour/SNP pact", Scottish Tory leader Ruth Davidson said Miliband and Sturgeon were already "at the altar and preparing to sign the register".