A British exit from the European Union would deter investment in the UK and threaten jobs, dozens of business leaders will warn.
Sky News has obtained the text of a letter expected to be published on Tuesday which will say that leaving the EU "would put the economy at risk".
It will praise the commitment secured by Prime Minister David Cameron for Europe to "deepen the single market and to sign off crucial international trade deals".
The text of the letter is understood to have been agreed with officials at 10 Downing Street, and will be the broadest vote of confidence from the business community so far for the reform package struck by Mr Cameron in Brussels on Friday night.
Sources said on Sunday that the chairmen or chief executives of between one-third and half of the companies in the FTSE-100 were likely to put their names to the letter.
Among the signatories are expected to be: Vittorio Colao, the Vodafone chief executive; Sir Roger Carr, BAE Systems (LSE: BA.L - news) chairman; Carolyn McCall, the easyJet boss; Gavin Patterson, BT Group (LSE: BT-A.L - news) chief executive; Sir Mike Rake, chairman of Worldpay; Sir Martin Sorrell, WPP Group chief executive; and Sir Andrew Witty, chief executive of GlaxoSmithKline (Other OTC: GLAXF - news) .
Other companies which have expressed concerns about Brexit in recent weeks include HSBC and Standard Life (Other OTC: SLFPF - news) , while Lord Blackwell, the chairman of taxpayer-backed Lloyds Banking Group (Other OTC: LLDTF - news) , has said he could support Britain leaving the EU.
According to one source who has seen this week's pro-EU letter, it will say: "We run businesses representing every sector and region of the United Kingdom.
"Together we employ hundreds of thousands of people across the country.
"Following the Prime Minister's renegotiation, we believe that Britain is better off staying in a reformed European Union.
"He has secured a commitment from the EU to reduce the burden of regulation, deepen the single market and to sign off crucial international trade deals.
"Businesses like ours need unrestricted access to the European market of 500m people in order to continue to grow, invest and create jobs.
"We believe that leaving the EU would deter investment and threaten jobs.
"It would put the economy at risk.
"Britain will be stronger, safer and better off remaining a member of the European Union."
Sky News revealed last week that Baroness Rock, a senior Conservative Party official, was helping to co-ordinate support for a pro-EU letter even before Mr Cameron's deal had been struck.
Reports that as many as 80 FTSE-100 companies would declare support for the 'In' campaign were dismissed as "nonsense" by one person close to Britain Stronger in Europe.
Some members of London's blue-chip share index are reluctant to disclose their views publicly, while a more limited number do not believe a Brexit would have a material impact on their business (Other OTC: UBGXF - news) .