Among new FTSE 100 companies represented are Barclays, Standard Life, Anglo American, Barratt Developments and Berkeley Group, although some leaders are signing in a personal capacity.

Other signatories include Ian Powell, PwC’s outgoing chairman and senior partner,and  David Sproul, senior partner and CEO, Deloitte UK.

The letter, published in the Times, says: ‘We know our firms are stronger in Europe. Our reasons are straightforward: businesses and their employees benefit massively from being able to trade inside the world's largest single market without barriers.’

The signatories claim that they would have to reset the terms of trade with Britain’s biggest market ‘from scratch’ after an exit from the EU and single market.

The letter states: ‘Smaller businesses and the people they employ are particularly vulnerable to any economic shock which could follow.’

For the other side, JCB chairman Lord Bamford has written to his company's 6,500 employees in the UK to explain why he favours a vote to leave the EU, stating that more than 53% of all UK exports go to non-EU nations and he is ‘very confident we can stand on our own two feet’.

Tate & Lyle Sugars has also written to staff saying leaving the EU would benefit the business. The letter, written by senior vice-president Gerald Mason, says that the EU has pushed up the firm's costs.

Manufacturing expert Sir James Dyson is another who opposes UK membership of the EU, describing voting to remain in the EU as ‘an act of national self-harm’.

However, research by BDO suggests the UK mid-market has been left cold by an EU referendum campaign which has failed to inform or inspire them.

The poll, which surveyed over 600 UK mid-market companies (companies with revenue of between £10m and £300m), found that, for nearly 90% of firms, the Remain or Leave campaigns had no impact on their opinions.  

A further 58% said the campaign was of low quality with just three firms out of 619 praising the high quality of the campaigns. 

The survey, conducted a few days ago, found that two thirds (65%) of the UK’s mid-market are backing Britain to remain part of Europe and 72% believe leaving the EU would make the economic environment less conducive to running a successful business.

Simon Michaels, managing partner, BDO said: ‘Exports and international trade are critical to the UK mid-market.  These firms are entrepreneurial, have high-growth targets and are focused on international expansion so it’s no surprise that, with only days to go until the vote, the majority of these businesses support remaining within the EU.'