Ed Miliband becoming prime minister would be a "catastrophe" for Britain, a leading business figure has warned amid fresh attacks on the Labour leader.
Boots boss Stefano Pessina said the Miliband's plans were "not helpful for business, not helpful for the country and in the end it probably won't be helpful for them".
It comes as Mr Miliband faced criticism from within the Labour fold, with donor John Mills attacking the direction the party is taking.
Mr Pessina, who oversaw the £46 million merger of Alliance Boots with American firm Walgreens and is now its acting chief executive, insisted Labour's plans for the country would be damaging.
He told The Sunday Telegraph: "If they acted as they speak, it would be a catastrophe.
"The problem is would they act that way or not? One thing is to threaten and to shout but it is completely different to be in charge and to manage the country day to day."
Mr Pessina also issued a warning over Britain's role in the European Union, insisting pulling out would be a "big mistake".
David Cameron has promised voters and in/out referendum by the end of 2017 if he is re-elected in May.
Mr Pessina told the paper: "Our main business is retail, so it's a local business to a certain extent. But I believe it would be big mistake for the UK to leave Europe, because the economy will suffer a lot and Europe is by the far most important commercial partner for the UK. It would have been like Scotland leaving the UK.
"These things are not really rationally conceivable today. Business leaders are convinced that the common sense of the British people will drive the right solution, which is staying in Europe."
Chancellor George Osborne was quick to seize on the comments made about Labour.
He said: "This is a clear warning from the head of one of Britain's biggest employers about the economic catastrophe the UK would suffer if Ed Miliband's policies were put into effect.
"The price of this catastrophe would be paid by families across the country who would see their jobs and incomes put at risk.
"It's another reminder that this election is a choice between Conservative competence and the chaos of all the alternatives."
But a Labour spokesman said: "Labour will cut and then freeze business rates for 1.5 million business properties including thousands of high street retailers.
"We will also ensure growing businesses get the lending and backing they need to succeed with a proper British Investment Bank and by boosting bank competition. In this way we will grow the economy so that it raises living standards for the many and not just a few at the top.
"The Tories would put British jobs, trade and investment at risk by flirting with exit from the European Union, which Mr Pessina has warned would be a 'big mistake'."
Earlier this week former health secretary Alan Milburn said Labour's focus on extra funding for the NHS without plans to deliver the reforms that the services needed could prove to be a "fatal mistake".
The warnings were endorsed by Mr Mills, founder of JML and Labour's biggest individual donor, who also underlined his criticism of the party's plans to introduce a mansion tax.
Mr Mills told the Mail on Sunday: "I agree with Milburn - if you look at the Continental model, they have a much greater mix of public and private provision."
He added: "I am not convinced of the case for a tax on expensive properties. And it certainly should not be hypothecated for the NHS. It would be much better to introduce extra council tax bands and spend the money on local needs, such as affordable housing."
Jason Cowley, editor of the left-leaning New Statesman, renewed his criticism of the party leader.
He told the Mail on Sunday the mood of Labour MPs is "not so much one of despair, it's worse than that - resignation", and claimed Labour appeared to be "willing itself to defeat".