The business support provided a boost for Cameron in his battle to win over a divided public and confront the many euroskeptics in his own party before Britain holds a referendum on whether to stay in the EU on June 23.

It came the day after the pound posted its biggest one-day loss in almost six years on concerns Britain might vote to leave the 28-member bloc.

Nearly 200 companies, among them 36 FTSE 100 firms, including telecoms group BT (BT.L), retailers Marks & Spencer (MKS.L) and Asda WMT.L, and oil firms Shell (RDSa.L) and BP (BP.L), signed the letter which put forward the economic merits of EU membership.

"Business needs unrestricted access to the European market of 500 million people in order to continue to grow, invest and create jobs," said the letter, published in the Times newspaper on Tuesday. "We believe that leaving the EU would deter investment, threaten jobs and put the economy at risk."