The bosses of more than a third of Britain's top companies on Tuesday urged voters to keep the country in the European Union, warning that an exit from the bloc would threaten jobs.
Some 198 business leaders including Roger Carr, chairman of BAE Systems, BP CEO Bob Dudley and Ron Dennis, chief of F1 team McLaren, wrote a joint letter published in the Times, backing Prime Minister David Cameron's deal to reform the EU.
"Following the prime minister's renegotiation, we believe that Britain is better off staying in a reformed European Union," they wrote, adding Cameron had secured important commitments on improving competitiveness within the bloc.
"We believe that leaving the EU would deter investment, threaten jobs and put the economy at risk," wrote the business chiefs, who between them employ around 1.2 million people.
"Britain will be stronger, safer and better off remaining a member of the EU," it concluded.
Separately, Nissan, which produced 450,000 vehicles in Britain last year, issued a statement saying it "made the most sense for jobs, trade and costs" to stay in the EU.
"For us, a position of stability is more positive than a collection of unknowns," said the statement. "However, this is ultimately a matter for the British people."
The backing comes as a boost for Cameron, who was rocked on Sunday by the decision of charismatic London mayor Boris Johnson to back a "Brexit" in the referendum on June 23.
Sterling fell to near seven-year lows against the dollar on Monday on concerns about the implications of a Brexit. While recovering somewhat the currency was still below last week's level on Tuesday.
Chairmen or chief executives of 36 companies from key share index FTSE 100, including national giants such as BT, Marks & Spencer, EasyJet, Burberry and Vodafone, all signed the letter.
Two leaders of US firm Goldman Sachs in Europe were also signatories.
- Tories 'more divided than ever' -
However, critics pointed out that many large employers such as retailers Tesco and Sainsbury's and banks RBS and Barclays had not signed the letter and accused Cameron of "bullying" businesses into supporting his position.
"The truth is that despite the bullying of a prime minister who has no real business experience, it is other normal commercial factors which will determine the continued success of British businesses to invest and grow," said Richard Tice, co-founder of pro-Brexit group Leave.EU.
Experts also pointed out that smaller businesses were less pro-EU than their larger counterparts.
"Small businesses are much less likely to export... so they would be less impacted by any changes to export patterns in the event of Brexit," Scott Corfe, Director, Centre for Economics and Business Research, told AFP.
Smaller businesses are less likely to be affected by a tightening of migration rules, given the local make-up of their staff, he added.
Supermarket giants Tesco and Sainsbury's both said they would not pass comment on the referendum, which will take place on June 23.
Despite Cameron's deal and polls showing marginal support for staying in the EU, bookmakers have slightly reduced the odds on Britain voting to leave, becoming the first ever EU member to quit the union.
Rating agency Moody's threatened to downgrade Britain's AA1 rating to "negative outlook" if the "Leave" camp won.
Six senior ministers have already publicly backed the "Leave" campaign and reports suggest around a third of Cameron's 330 lawmakers could do likewise.
Former Conservative leader William Hague on Tuesday warned that the splits in the party over the issue could fester for years.
"Not since the fierce disagreements over the euro in the 1990s have Conservatives so strongly opposed each other in public on a fundamental issue," he wrote in the Daily Telegraph.
"The party is more evenly divided than it ever was then. A sustained battle within a party can open wounds that take a generation to heal."