The UK should leave the European Union unless fundamental reforms are secured by David Cameron in his renegotiation process, a major report by a pressure group has concluded.
The Business for Britain group's report concluded that the current terms of the UK's membership are "unacceptable" and holding the country back.
The group's 1,032 page document found that the amount the UK got back from the EU was just 49p from every £1 paid into Brussels coffers since the eurozone crisis began in 2009 and claimed that the country's influence in European institutions was declining.
Business for Britain's chief executive Matthew Elliott said: "Britain would be better placed to attract investment and cut harmful regulations if it had a looser relationship with the EU, and it would also be better protected from a eurozone that will either fully integrate or disintegrate.
"Britain is in a win-win situation; a scenario which boosts the chances of securing the 'fundamental change' in Britain's EU relationship that would deliver the looser, trade based relationship with the EU which business wants."
The analysis, written by senior business figures including John Moynihan and John Mills, says treaty change is "essential", the arrangement must be more like a free trade area than a political union, and powers must be reclaimed.
"The current terms of membership are unacceptable and are only going to get worse," the report said.
"Britain's economy has been held back by the demands of a regional bloc whose economy has become increasingly inward-looking and uncompetitive, and whose policies, not least the flawed single currency, have created the conditions for ongoing, persistent economic crisis."
The group concluded that if the EU was "intransigent" about the UK's demands for a looser arrangement then "given the fundamental problems that define our membership, we believe that Britain should vote to leave".
The UK "has nothing to fear from such a vote and, indeed, much to gain", the report said.
Leaving the EU would increase Britain's influence over its own affairs and "make politics meaningful again".
The report questioned the value that the UK was getting from its contributions to the EU budget.
The report found that the "average return" from 1973 to 2013 had been 68% - so the UK has received back roughly 68p in grants and rebates for every £1 it has paid into the EU. But since 2009 the return has only been 49%.
"Leaving the EU would give the UK a major opportunity to make substantial savings," the report said.
"The UK's contributions to the EU Budget have been, for many years, on an upward trajectory, punctuated by incidents like demands for surcharges.
"There are many options available for the UK outside the EU to continue to fund those EU programmes it benefits from (gaining more control in the process over where its contributions flow to), and leaving offers a chance to replace opaque and unsatisfactory EU funding schemes with a more transparent and accountable system.
"From a financial perspective, the UK, as a long-term net contributor to the Union, would lose little from leaving the EU and would have much to gain.
"Staying, by contrast, holds serious risks. Were the eurozone to encounter further financial difficulties it, via budgetary exposure, raises the real and dire prospect of UK contributions increasing even more."