Some of the country's most prominent business leaders have accused Eurosceptic MPs of "putting politics before economics" by calling for Britain's withdrawal from the European Union.
The group, which includes the chairmen of BT, Deloitte and Lloyds along with Sir Martin Sorrell and Sir Richard Branson, said Britain's business interests and economy can only benefit from playing a central role in the EU.
In a letter to The Independent, they called for David Cameron to "strengthen and deepen" the European single market, adding: "The economic case to stay in the EU is overwhelming."
David Cameron sent an email to members of his Conservative Party urging unity.
The Tory grandee Lord Howe has accused the Prime Minister of 'losing control' of his party as divisions over Europe come to the fore.
The Conservative Party has moved to head off a row with grassroots activists after a Tory ally reportedly described them as "loons".