David Cameron received a timely boost to his campaign to keep Britain in the European Union after business leaders signed a letter in which they highlighted the damaging implication a Brexit would have on Britain's economy.

The 200 signatories of the letter addressed to The Times included more than a third of Britain's 100 largest companies, including Asda, BT, Vodafone and Marks & Spencer, although the likes of Barclays, Sainsbury's and Tesco declined to sign the document, with the latter saying the referendum was "a matter for the British people to decide".

However, while a number of British businesses have thrown their weight behind the campaign to keep Britain in the 28-country bloc, a number of high-profile business figures have made it clear they will vote "leave" on 23 June.

John Mills, millionaire Labour donor and founder of John Mills Limited (JML), is among the supporters of the Vote Leave campaign, while Joe Foster and John Caudwell, the founders of Reebok and Phone 4U respectively have also backed the Brexit movement.

John Timpson, the CEO and owner of retailer Timpson, Luke Johnson, the chairman of cafe chain Patisserie Valerie, and Nigel Wilson, the CEO of Legal & General, have all expressed themselves in favour of Britain leaving the EU. Oliver Hemsley, CEO of Numis Securities, and Crispin Odey, boss of leading hedge fund Odey Asset Management, have also backed the leave campaign.

Speaking back in 2013, Jim O'Neill, the former chairman of Goldman Sachs's asset management business, indicated the UK should not be afraid of explore the opportunities that are arising outside the union.

"We should not be scared of leaving it [the European Union] and exploring a world without it," he said. "The opportunities that are arising from the dramatically changing world are huge and I don't think quite a lot of people in our area, never mind people in Brussels, are that interested or understand it."

At the time, however, the possibility of a EU referendum was not even on the cards and O'Neill has since indicated his comments should not be interpreted as pro-Brexit.

On 17 February, 80 business and community leaders, including Pasha Khandaker, president of the UK Bangladesh Caterers Association UK, Moni Varma, owner of rice suppliers Veetee, and Tariq Usmani, CEO of Henley Homes, wrote to the prime minister to highlight Britain's EU membership was damaging trade with the rest of the world.

"As long as Britain's trade policy is controlled by the EU, we cannot sign bilateral free trade agreements with Pakistan, India, Bangladesh, Australia, New Zealand or for that matter any other non-EU state," the latter said.

"Vested interests on the continent sustain a relatively protectionist policy. We have to apply the EU's common external tariff to exports from Commonwealth countries – hurting consumers here as well as producers there.