The chairman of insurer Standard Life has told the CityWeek conference it would be "disastrous for London and the UK" if Britain were to leave the single market.
Sir Gerry Grimstone said:
Why on earth would we not want to be part of one of the biggest markets in the world? Our whole energy should be devoted to that.
This is not the first time senior management at the company have warned of the consequences of Brexit. In February, Standard Life Investments chief executive Keith Skeoch told journalists that the "biggest risk from the election is the possibility of an EU referendum".
He added that Brexit "would cause a great deal of uncertainty". Responding to Grimestone's comments, Robert Oxley, campaign director of the Eurosceptic group Business for Britain said:
Gerry Grimstone rightly supports an EU referendum, but is wrong to join in the scaremongering that life outside of the EU would be disastrous for the UK. Britain inside a significantly reformed EU, or outside with the right deal would be in a far better position than if we stick with the status quo or only pursue timid reforms.
Grimstone's comments come only a day after a report from Open Europe argued in the worst case scenario UK GDP could shrink by 2.2 per cent by 2030 following a Brexit.
The report makes clear, however, that the more likely outcomes of life after Brexit are between a net cost of 0.8 per cent of GDP or a net benefit of 0.6 per cent.
As things stand at the moment, Open Europe's Brexit barometer puts the chances of the UK leaving the EU in the next parliament at just 17 per cent.
David Cameron has promised a renegotiation of Britain's terms of membership followed by an in-out referendum. Labour leader Ed Miliband by contrast has stated his commitment to EU membership and ruled out any possibility of a referendum should Labour win in May.