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David Cameron's pledge to hold a referendum on Britain's EU membership is "a clear and present danger" to jobs and business, Ed Miliband has warned.
The Labour leader has outlined his party's business manifesto, which includes a promise to "return Britain to a leadership role" in Brussels.
The PM has pledged an in/out referendum by the end of 2017, while the Lib Dems are "unambiguous" in their EU support.
Meanwhile, some firms criticised the use of their quotes in a Labour advert.
Mr Miliband used the first official day of election campaigning to unveil his party's Better Plan for Business during a speech at Bloomberg's European headquarters in London.
'Over-stepped the mark'
To reinforce Labour's business message, the party also took out a full-page advertisement in the Financial Times, setting out its determination to "put the interests of Britain and British business first rather than risk an EU exit".
But a spokesman for Siemens said Labour had "over-stepped the mark" by quoting its chief executive, Juergen Maier, in the advert.
In it, Mr Maier says: "The prospect of a referendum that may or may not happen, at a date yet to be decided upon, with a choice between two unknown options, is profoundly worrying for business leaders."
While the quote is not disputed, a spokeswoman said the firm would be contacting Labour to express its displeasure.
"We did not give them permission [to use the quote]. We did not know about this. We are an apolitical organisation," she said.
Analysis by Business Editor Kamal Ahmed
Labour has quoted a number of pro-EU company leaders in the full page advert the party has taken out in the FT today.
"We would be devastated - as a company that loves London and wants to be in London - if Britain were out of Europe," is the view of Andrew Mackenzie, chief executive of the mining giant, BHP Billiton.
My Labour sources were being very careful to explain last night that the business leaders quoted in the advert were not endorsing Labour.
They were simply being quoted to show the strength of feeling about Europe.
What is interesting is that Labour has chosen the first day of the election campaign to focus on an area - business - some see as a weakness for the party.
Kellogg's said "eyebrows were raised internally" when the company was told its head of UK and EU operations, Jonathan Myers, would be quoted. But a spokesman said it was too late to have the remarks removed by the time they were told about them on Sunday afternoon.
Asked about the quotes, Mr Miliband said: "We've simply quoted public statements by these businesses about the place of Britain in the European Union.
"Lots of businesses all round this country aren't necessarily going to be supporting Labour or the Conservatives but they do have a very strong view about our place in the EU."
'Start a revolution'
Meanwhile, Labour's economic claims were also called into question by one of its leading donors, Dr Assem Allam, who told the Daily Telegraph that its plans for a "mansion tax" and a rise in the top rate of tax were "alienating" wealth-creators.
Mr Miliband retorted that unlike the Conservatives, Labour "donors will have particular views on things - we don't always do what our donors say".
The Labour leader launched his party's manifesto on the eve of a party political broadcast by Hollywood actor and Sherlock star Martin Freeman who is set to endorse Labour's values of "community, compassion and decency".
The 22-page manifesto promises to cut and freeze business rates for more than 1.5 million small business properties, while at the same time creating an independent National Infrastructure Commission and a British Investment Bank that will support a network of regional banks to boost competition on the High Street.
Mr Miliband also promised "to start a revolution in vocational education" that will ensure all young people study English and maths to 18, with a guaranteed apprenticeship for those that work hard and get the grades.
Mr Balls insisted the manifesto would not include any pledges that required "additional borrowing" - but the Conservatives warned that a Labour government would raise taxes for every working family in Britain by £3,028.
'Banging on about Europe'
But it was the EU that Mr Miliband focused on - and what he sees as the consequences of an in/out EU referendum.
Pledging to return Britain to "a leadership role" in a reformed EU, he said: "There could be nothing worse for our country or for our great exporting businesses than playing political games with our membership of the European Union - I'm not going to do it."
He said David Cameron "came to power promising to stop his party 'banging on about Europe'. Now if he has his way, he'll have us inside Europe banging on the door to leave or, even worse, outside Europe - banging on the door to be let back in."
"It's simply the wrong direction for our country," he said.
"We'll show strong leadership to get the best for Britain. We won't condemn this country to years of uncertainty, years of insecurity, by threatening our European future."
But Simon Walker, director general of the Institute of Directors, said his members thought it was "sensible" to hold an EU referendum based on reforms, and he warned Labour "there's more to business than just Europe".
The Conservatives want to renegotiate Britain's membership of the EU before holding a referendum.
The Lib Dems will hold a poll if more powers are transferred to Brussels.
But UKIP says while it wants Britain to trade with Europe, it should make its own laws.