Only 36 of the biggest 100 companies in the UK have signed a letter, published in the Times, backing David Cameron’s position that the UK should stay inside the European Union.
Signees include EasyJet, the defence company BAE systems, the chief executives of Heathrow and Gatwick airports and the oil group Shell.
They say that leaving the EU would threaten jobs and deter investment in the UK.
Tesco, Sainsbury’s, Morrisons, Lloyds and Barclays are among the absent FTSE 100 companies.
Nigel Wilson, chief executive of Legal and General, Alison Brittain, the chief executive of Whitbread, Jeff Fairburn, chief executive of Persimmon, and Robert Noel, chief executive of Land Securities, all refused to sign the letter.
All four provide regular business advice to David Cameron as members of the prime minister's business advisory group.
Downing Street has dismissed suggestions that far fewer FTSE bosses were willing to sign up than was hoped, putting it down to internal bureaucracy, including certain procedures that companies must go through before they can back a political campaign.
Some of the 64 FTSE 100 companies that didn’t sign have revealed why:
Backing Europe will deter customers, according to Sainsbury’s, Morrisons and Tesco.
Tesco said in a statement: “The referendum on EU membership is a decision for the people of Britain. Whatever that decision is, our focus will continue to be on serving customers.”
Companies are keen to be seen as "neutral" to avoid offending customers on either side of the debate.
“We have thousands of employees and customers and no doubt there are strong opinions on both sides. The board has discussed it, and Persimmon has decided to remain firmly neutral," a spokesman for Persimmon said.
Sainsbury’s said it was an “apolitical organisation” and that the referendum is a matter for the people.
Some companies haven't made up their mind yet. A spokesman from Legal and General told the Independent that the board had always planned to discuss the EU referendum once the PM has presented his deal.
The letter was too general. That's the reason that Robert Noel, chief executive of Land Securities, decided not to sign. He is personally pro-EU, but it was felt that since Land Securities only operates in a domestic setting, the letter was too general to sign.
What has the EU ever done for us?
What has the EU ever done for us?
1/7 1. It gives you freedom to live, work and retire anywhere in Europe
As a member of the EU, UK citizens benefit from freedom of movement across the continent. Considered one of the so-called four pillars of the European Union, this freedom allows all EU citizens to live, work and travel in other member states.
2/7 2. It sustains millions of jobs
A report by the Centre for Economics and Business Research, released in October 2015, suggested 3.1 million British jobs were linked to the UK’s exports to the EU.
3/7 3. Your holiday is much easier - and safer
Freedom to travel is one of the most exercised benefits of EU membership, with Britons having made 31 million visits to the EU in 2014 alone. But a lot of the benefits of being an EU citizen are either taken for granted or go unnoticed.
4/7 4. It means you're less likely to get ripped off
Consumer protection is a key benefit of the EU’s single market, and ensures members of the British public receive equal consumer rights when shopping anywhere in Europe.
5/7 5. It offers greater protection from terrorists, paedophiles, people traffickers and cyber-crime
Another example of a lesser-known advantage of EU membership is the benefit of cross-country coordination and cooperation in the fight against crime.
6/7 6. Our businesses depend on it
According to 71% of all members of the Confederation of British Influence (CBI), and 67 per cent of small and medium-sized enterprises (SMEs), the EU has had an overall positive impact on their business.
7/7 7. We have greater influence
Robin Niblett, Director of think-tank Chatham House, stated in a report published last year: “For a mid-sized country like the UK, which will never again be economically dominant either globally or regionally, and whose diplomatic and military resources are declining in relative terms, being a major player in a strong regional institution can offer a critical lever for international influence.
The vote is a matter for the British people, according to Lloyds Banking Group.
The chairman of Lloyds, Lord Blackwell, has spoken out about the need to reform the EU.
Then there is the issue of company policy. The chairman of Barclays, John McFarlane, has said that signing the letter would be “against company policy”.
The letter appeared in the Times after David Cameron presented the terms of his EU agreement to the House of Commons.
A referendum will be held on June 23.