British businesses say leaving Europe would hurt

Vodafone CEO: 'We want Britain to remain in Europe'
Vodafone CEO: 'We want Britain to remain in Europe'

What if Britain votes to leave the European Union?

A so-called 'Brexit' (British exit) from the EU would trigger a host of unknown consequences, and the country's boardrooms are getting increasingly worried.

A survey of the top 350 publicly traded U.K. companies showed 70% now say leaving the EU would damage their business, up from 63% last summer.

And 61% say being inside the EU has a positive effect. That's up from 42% in 2013, when the idea of an exit was more remote.

The survey was conducted by ICSA, an association of board members in charge of compliance and risk, and first published by the Financial Times.

Prime Minister David Cameron has vowed to hold a referendum by the end of 2017 on whether Britain should leave the block of 28 countries and 500 million people. The vote became a certainty after Cameron's Conservative Party won a clear majority in May's general election.

He's negotiating with other countries on reforms to rules on welfare and immigration that, he hopes, will appease members of his party that want Britain to leave.

Supporters of EU membership say it brings significant economic gains -- worth as much as £78 billion ($118 billion) a year, according to the Confederation of British Industry. Opponents argue Britain needs to reclaim the right to make its own rules and regulations on business.

If Britain left, it would join non-EU countries such as Norway and Switzerland who actively trade with the EU but have no say on how it operates.

Related: Britain's four demands to say in Europe

In the survey, only 2% of U.K. firms say their business would benefit from leaving the EU.

Yet despite the growing concern, only a quarter of boards are considering the consequences of a 'Brexit.'

Former Marks & Spencer (MAKSY) chairman Stuart Rose is leading the 'Britain Stronger in Europe' campaign. He's asked for British business to be vocal on a possible 'Brexit,' but only 4% said in the survey they are prepared publicly to take a stand, for or against.

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