Manufacturers have sent the Government a strong message that the UK should remain part of the European Union to help secure jobs and economic growth.
A report by the EEF said membership of the EU was central to businesses and their investment plans.
A survey of 400 EEF members found that 85% would vote to remain in the union if there was a vote now.
The manufacturers organisation has written to the leaders of the three main political parties saying that the economic benefits of membership far outweigh concerns about regulation or the repatriation of powers.
A third of those polled said they would be less likely to increase investment in Britain if the country was outside the EU.
Two thirds believed they would have to make significant changes to their business plans if the UK exits.
Terry Scuoler, chief executive of the EEF, said Britain should not "gamble" on its future in Europe, adding: "It is naive to think we can simply pull up the drawbridge and carry on as normal. The debate must move on to how we can make Europe work to support jobs, growth and higher living standards.
"We need to focus on the real prize - how we can get Europe to work better supporting companies that are looking to sell into the EU, to export to new markets in the rest of the world and develop new products and services.
"We must be at the centre of the change we want to see in Europe, to help secure the prize and ensure we share in it. Billions of pounds of trading opportunities are at stake and we must keep the focus - and the wider debate - on these opportunities which will unleash growth in our economy now and in the longer term."
Business Secretary Vince Cable said: "Here is one of our leading, non-political employer organisations - the voice of the makers of Britain - making it unambiguously clear that it would be damaging to growth if we left or even formulated proposals to leave the EU.
"Our focus must remain on making the case for Britain being strong in Europe as millions of British jobs depend on it."