In Britain's most unpredictable election since the 1970s, opinion polls show neither Prime Minister David Cameron's Conservatives or the opposition Labour Party are likely to win an outright majority.

 The Scottish National Party (SNP), who have rebounded after leading a failed bid for independence last year, are set to virtually wipe out Labour in Scotland and could end up as kingmakers in the event of a hung parliament.

"The sharp point for this election is the fact we have an opportunity to bring austerity to an end, and that will be the top priority for the SNP," said the SNP's John Swinney, Deputy First Minister of Scotland.

 The SNP will use its manifesto launch in Edinburgh on Monday to set out an alternative to austerity, proposing real terms public spending increases of 0.5 percent per year and saying it would invest in jobs, economic growth, and public services.

 It wants to scrap the renewal of Britain's Trident nuclear deterrent, something it says would free up 3 billion pounds  ($4.5 billion) a year to spend on health, education and childcare.

 The SNP has repeatedly called on Labour to work with it to block Cameron's Conservatives from returning to power. Labour leader Ed Miliband has ruled out a formal coalition with the SNP, but not a looser vote-by-vote arrangement.

 Writing in the Telegraph, Conservative London Mayor Boris Johnson warned it would be "nuts" to hand the SNP a position of power over the government of a country they sought to break up.

 "It would not normally occur to you to interview a convicted jewel thief for the post of custodian of the Tower of London," he wrote, referring to the home of Britain's Crown Jewels.

"They want to end Britain to cause a constitutional upheaval that would gravely weaken this country."

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