David Cameron is putting the nation's future prosperity in danger by flirting with the possibility of Britain leaving the European Union, Ed Miliband will claim.
The UK is being left increasingly isolated and is now on a "conveyor belt towards exit with no idea how to get off", the Labour leader is set to warn.
In a speech to the CBI on Monday, he will accuse the Prime Minister of putting millions of jobs at risk.
Mr Miliband will also pledge not to play "political games" with Britain's EU membership and insist Labour has a credible and clear plan to secure reforms.
He will say: "There are some people in our country who advocate exit from the EU. There are others who flirt with it, thinking they can do so without consequence. Both are equally dangerous.
"Giving succour to the argument that the real answer is leaving the EU, or contemplating it, simply drags us closer to exit.
"And every nod and wink to those who want to leave sends a message to potential investors in our country that we are not open for business.
"It is a betrayal of our national interest. It is a clear and present danger to our future prosperity.
"The way to overcome these false solutions is not to pander to them. Not to posture about finding ways of accommodating them. Not to make promises everyone knows you cannot keep."
It comes after George Osborne was forced to defend a deal that will leave Britain paying an extra £850 million towards the European Union budget following accusations he resorted to "smoke and mirrors".
Critics claimed the reduction in the £1.7 billion bill had been achieved by bringing forward a rebate to which the UK would have been entitled anyway.
But the Chancellor insisted there had been "real doubt" over whether that clawback would apply to the surcharge.
Mr Cameron has promised a renegotiation of Britain's relationship with Brussels followed by an in/out referendum if re-elected next year.
But the Prime Minister's drive to curb immigration from other EU states has been met with an icy response from other countries, including Germany, Finland and Sweden.
Mr Miliband will say Britain must build alliances rather than burn them to help change the EU so it works for Britain.
"Under a Labour government, there will be no further transfer of powers to the European Union without the explicit consent of the British people in an in/out referendum," he will say.
"But we have seen over the last couple of years that trying to use exit as a threat has simply weakened our influence, not strengthened it.
"A country making ever-more incoherent demands, ever-more isolated from its partners, is one on the conveyor belt towards exit with no idea how to get off.
"We have seen it again with the controversy over the EU budget: banging the table one day, then paying up the next.
"Instead of posturing, Britain should be forging alliances now to make real reform happen and taking our rightful place in forging the future of Europe."
He will add: "I make you this promise: if I am prime minister I will never risk your businesses, British jobs or British prosperity by playing political games with our membership of the European Union.
"We will reform the European Union, not threaten to leave, not pretend that is the answer, not playing fast and loose with Britain's future, Britain's prosperity and your businesses."