The bosses of a third of Britain's biggest companies warned on Tuesday that an exit from the European Union would put the economy at risk and threaten jobs.
In a letter to the Times newspaper, bosses from big employers including telecoms group BT, retailers Marks & Spencer and Asda and oil firm BP joined forces to argue that access to the EU's single market enabled firms to grow and create jobs.
The letter, signed by 36 of the bosses of FTSE 100 companies, is likely to be seized on by Prime Minister David Cameron who is battling to persuade Britons to remain in the 28-member bloc in a June 23 referendum.
On Monday, the pound posted its biggest one-day loss in almost six years on concerns that Britain could vote to leave the bloc after the influential London mayor, Boris Johnson, said he would campaign for a British exit, or Brexit.
"Business needs unrestricted access to the European market of 500 million people in order to continue to grow, invest and create jobs," the letter signed by almost 200 business leaders said.
"We believe that leaving the EU would deter investment, threaten jobs and put the economy at risk. Britain will be stronger, safer and better off remaining a member of the EU."
Those signing the letter included the chief executives or chairman of 36 FTSE 100 companies however many large firms opted not to sign the letter, preferring instead to stay neutral in the highly charged debate.
The move, organized by the Britain Stronger in Europe campaign, with the support of Cameron's Downing Street office, echoes similar moves made by big business in the run up to the 2014 Scotland independence referendum, which helped to keep Scots in the United Kingdom.