The gloves will come off on the first day of the general election fight as Ed Miliband’s campaign launch clashes with an aggressive Conservative attempt to undermine Labour’s spending plans.

Addressing a rally in Manchester, the Labour leader will say his party offers “hope, not falsehood”, while listing his priorities as putting working people first, dealing with the deficit and protecting the NHS.

Just over an hour after Miliband’s big speech, the Conservatives will try to overshadow it by lining up cabinet big-hitters to make claims based on Treasury figures about the cost of Labour’s 2015-16 spending plans in areas including education, media, tax and home affairs.

With the political atmosphere heating up, Ukip will also begin to flesh out its policies in more detail within days, with its leader, Nigel Farage, promising a series of speeches on areas outside his usual topics of the EU and immigration.

As the Tories fight to win back voters who have defected to Ukip, the prime minister hinted on Sunday that his party could be open to holding an earlier European Union referendum and twice refused to speculate on the possibility of a deal with Farage’s party after the election.

A senior Tory source later played down the EU plan, saying the difficulty of negotiations would mean it could probably only be done a few months early.

The rival events and speculation about alliances are early skirmishes in what is likely to be a brutal battle for dominance of the airwaves and newspaper column inches in the months ahead.

Miliband will kick off the day with a speech in the morning in which he challenges activists to engage 4 million voters in conversation and a promise to hold his own “people’s question time” events weekly for the next four months.

In his speech, he attacks the Conservatives, and will say: “They’re the pessimists about what is achievable for Britain … between now and the election, they will tell you that change isn’t possible, just as the pessimists have always done down the years. But we don’t agree. We have rebuilt before as a country, in the face of even greater challenges, and we can rebuild again.”

At almost the same time, the Liberal Democrat leader, Nick Clegg, will weigh in to the fray with a press conference in which he will claim voters need to back his party if they want another coalition. Presenting the Lib Dems as the occupants of the centre ground, he will say Conservatives are like mobile phone salesmen who signed you up to a contract then cut the number of calls you could make, while Labour is like an ex-boyfriend leaving late-night voicemails asking for one more chance.

Minutes afterwards, the Conservatives will line up five senior Tory cabinet ministers – George Osborne, Nicky Morgan, Theresa May, Sajid Javid and William Hague – to argue that Labour’s policies amount to billions of pounds of extra spending in the first year of a new parliament.

They will say their claims are based on a Treasury analysis of opposition spending, which will be published on the government’s official website, as well as “other reliable sources”.

“The evidence we will produce shows that Labour have not demonstrated the fiscal discipline or economic competence that earns an opposition the credibility to form a government,” a Tory spokesman said.

Labour acknowledges that the Treasury is allowed to make such calculations but Ed Balls, the shadow chancellor, has written to Nicholas Macpherson, the permanent secretary at the Treasury, to demand a right to examine the underlying assumptions of such figures.

The party is furious that Cameron claimed over the weekend Labour would cost the country an extra £13.5bn in debt interest, based on Treasury figures, saying this was founded on “blatantly false and politically motivated assumptions” provided by Tory advisers.

Labour has a stubborn, though narrow, lead in the vast majority of surveys but both Cameron and Miliband concede the election is going to be an extremely close contest. Although this is the first proper day of campaigning after the Christmas break, the Tories and Labour have already locked horns on the economy and NHS – the two issues likely to dominate the race.

Labour claimed the NHS would not survive another Tory government, while the Conservatives said a Labour government would lead to economic chaos.

Chris Leslie, Labour’s shadow chief secretary to the Treasury, rubbished the Tory claims. “It is David Cameron and George Osborne who have made over £7bn of unfunded tax promises,” he said. “These could only be paid for by another Tory VAT rise, even deeper cuts to public services or both.

“Labour has made no unfunded commitments. In fact, the Institute for Fiscal Studies said last month that we had the most cautious approach and, unlike the parties, had promised no net giveaways.

“If the Tories wanted a serious debate they would not be blocking our proposal to allow the Office for Budget Responsibility to independently audit the manifestos of the main parties. Instead it’s clear they want to carry on spreading smears about Labour while avoiding scrutiny of their own plans.”

The two parties have also both launched their first campaign posters, both of which have been challenged over their accuracy. A Conservative party advertisement said the deficit had halved, when in fact it is only down in cash terms by about a third.

The Tories hit back at Labour’s poster showing an airbrushed picture of Cameron, next to a slogan saying public spending would fall to 1930s levels. Critics argued this too should have had the caveat “as a proportion of GDP”.

Labour’s claims on the poster forced Cameron to defend the scale of his party’s plans to slash public spending by £25bn, in an interview with the BBC’s Andrew Marr Show on Sunday.

While Labour portrayed the Tories as economic extremists making cuts for ideological reasons, Cameron insisted these “adjustments” would be moderate, sensible and reasonable.

He said it was Miliband’s plans that would lead to “real poverty” and claimed the prospect of Labour and the Liberal Democrats running the economy was genuinely frightening.

“Why do we need to do this? It is not some ideological obsession. It is not because we’re desiccated accountants. It is because I think it’s right for our country, it’s right for future generations, and at the end of seven years of economic growth, by 2018 we should start not borrowing but putting aside money for a rainy day, to be saving money for when storms in the future might hit,” he said.

Following Lib Dem claims about the potential impact of defence cuts, he said the overall manpower of the armed forces would not decline further. But he would not say whether Britain would continue to meet its Nato obligation to spend 2% of GDP on the military.