ED Miliband has branded as "completely false" Tory claims that a Labour government in only its first year in power would create a £21bn black hole in Britain's finances, leading to higher borrowing and more tax rises.

The bitter clash marked the first 24 hours of the 2015 General Election campaign proper with the UK Labour leader insisting the policy differences between his party and David Cameron's Conservatives meant the May 7 poll was a "once in a generation" chance to rebuild a better nation.

In Edinburgh, Jim Murphy, the new Scottish Labour leader, promised the party would recruit 1000 extra nurses. The £30m cost would come from Scotland's share of funds generated by Labour's proposed mansion tax, the annual levy on homes - mainly in London - worth more than £2m.

Meantime, Nick Clegg said voting for the Liberal Democrats was the only way to support "the national interest against petty populism".

At his regular Westminster press conference, the Deputy Prime Minister personally pledged to campaign against an "arrogant" Alex Salmond on the streets of Gordon but refused to rule out entering a rainbow coalition with the SNP after the election.

The Lib Dem leader also threw down a pre-election gauntlet to the other parties by pledging to fill a £30bn funding black hole in the NHS in England by ploughing the proceeds of future economic growth into public services.

Launching what will be a four-month election campaign, Mr Miliband in a keynote speech in Salford made clear Labour would reduce public spending but would cut the £91bn budget deficit "responsibly", including through higher taxes on the wealthiest; there would be no manifesto promises funded by additional borrowing, he stressed.

The UK Labour leader argued that amid the sound and fury of electioneering, the contest would come down to a simple message about "who we are, how we want to live together and how we succeed as a nation".

He stressed: "This is nothing less than a once-in-a-generation fight about who our country works for. It is a choice between a Tory plan where only a few at the top can succeed and our public services are threatened. Or a Labour plan that puts working people first, deals with the deficit and protects our NHS."

The party chose the health service for its first campaign poster, warning voters that the NHS as we know it could not survive another five years of Tory management. Mr Miliband denied the Tory charge of "scaremongering".

Mr Miliband insisted his party offered "hope not falsehoods" as he dismissed Tory claims Labour had made £20.7bn in unfunded spending commitments.

At a campaign event in central London, George Osborne, flanked by four Cabinet colleagues, published an 82-page document, drawn up by Treasury officials, which claimed to show the Opposition had made unfunded spending commitments for 2015/16 that would require an extra £1200 of public borrowing for every working household.

The evidence, insisted the Chancellor, showed Labour had learnt nothing and posed a risk to Britain's economic recovery; the choice was a simple one between "Conservative competence or Labour chaos".

But while Mr Osborne claimed some of the figures used were from Labour announcements and conference speeches, he also accepted some were based on assumptions; applying a "reasonableness test" that when the Opposition had criticised a Coalition policy, this meant it would reverse it.

But Ed Balls laid into the Conservative document, dismissing it as a "dodgy Tory dossier...riddled with untruths and errors on every page".

He denied the Tory assumption that Labour would reverse almost £5bn of spending cuts to the police, hospitals and local councils in England as well as to the armed forces.

Also it emerged a pledge by Labour to ban putting waste food into landfill, that would cost £477m to the Exchequer in lost revenue, made in 2013 had been dropped last year.

Earlier in a separate development, Mr Miliband suggested to some a shift on the Faslane-based nuclear deterrent, stressing how, while still a multilateralist, he wanted the "least-cost deterrent" possible.

The SNP is totally opposed to renewing Trident but the Labour leader's remarks potentially open up the way to a deal with the Liberal Democrats, who want to retain the submarine-based deterrent but not in a continual, at-sea deployment like the Tories do.

Last night, Labour insisted there had been no change in policy and it still wanted a "minimum, credible independent nuclear deterrent...delivered at the least cost possible".

Elsewhere, William Hague, the Commons Leader, suggested the UK Government would this week make an announcement on the devolution of Corporation Tax to Northern Ireland, opening up another political front as, once this happens, the SNP will demand a similar move for Scotland.