European aerospace group Airbus, one of Britain's largest employers, has voiced concerns over the possibility of the country leaving the European Union, saying the benefits of an alternative economic model needed to be proven.

The Franco-German company, that employs 17,000 workers in Britain, is the latest large foreign investor to say it favours Britain continuing its membership of the 28-member trading bloc.

Car giants Ford and Japan's Nissan have said they would have to re-evaluate their operations if Britain pulled out of the EU in a proposed referendum.

Airbus, formerly known as EADS, employs most of its staff in Bristol and north Wales where the company's passenger plane wings are made and assembled.

"Airbus Group, we note, would never have achieved its success to date without a working and effective partnership of countries and companies within Europe, which only collectively can deliver the scale required to be globally successful," UK Chief Executive Robin Southwell will tell attendees at a company event late on Tuesday.

"Any other economic model which seeks or offers to change the dynamic and advantageous characteristics which we presently enjoy - and believe are optimal to our delivering sustained growth and employment - would need to specifically address this quite proper challenge in a detailed and compelling manner."

Prime Minister David Cameron promised voters he would renegotiate the terms of Britain's EU membership before holding an in-out referendum by 2017 if his ruling Conservatives were returned to power after elections due in May 2015.

The centre-right party is trailing in the polls and faces a rising threat from the small, anti-EU UK Independence Party (UKIP).

Last month Business Secretary Vince Cable told an economic conference the chances of Britain leaving the EU were "very remote" with a five percent chance of happening.

Citigroup joins warning

The corporate sector warnings for Britain to stay in the EU have become bigger and louder.

On Tuesday (21 January) by Jim Cowles, chief executive officer for Europe, Middle East and Africa at Citigroup, told the Financial Times newspaper that there was "mounting concern" among clients about their ability to use the United Kingdom as a regional hub if the country exits the EU.

"It is not that international companies will stop investing in Britain, but their investment just will not be at the scale we have become accustomed to," Cowles told the newspaper.

He added that the existing trade arrangements will be at risk and Britain will have no influence in making the rules in the future.

According to the Financial Times, the British Bankers' Association has also sent a submission to a Treasury review of powers between the United Kingdom and Germany. The submission said that a single market for financial services is a significant factor in the success of the United Kingdom as a financial centre, and of considerable value to the UK economy.

The London-based Centre for European Reform (CER) predicted last week that Britain would struggle to maintain trade with other EU member states - now 54 percent of goods trade - if it left the bloc.