Bosses at more than a third of Britain's biggest business have come out in support of the campaign for the country to remain in the European Union.
Asda, BT, Marks & Spencer, Kingfisher and Vodafone chiefs have backed a letter warning of the risks to the economy of quitting the 28-member bloc.
Chairmen or chief executives of 36 FTSE 100 companies say a Brexit would "deter investment and threaten jobs".
Among the signatories are the chief executives of Heathrow and Gatwick but the total number of firms falls short of the 80 or 50 that it had previously been suggested would sign.
In a letter to The Times, they wrote: "Business needs unrestricted access to the European market of 500 million 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."
The letter includes some notable absences, such as Tesco, Sainsbury's and Barclays.
Among those who have signed are Tory donors and figures who have accepted government roles under Mr Cameron's premiership, according to The Times.
Roland Rudd, treasurer of Britain Stronger in Europe, which organised the letter with No 10's support, told the newspaper: "This is the single biggest number of business leaders who have been willing to support staying in a reformed EU.
"What is also striking is the number who have done so on behalf of their companies as well as in a personal capacity."
David Cameron earlier defended using a Downing Street civil servant to lobby businesses to support the pro-EU campaign after MPs heard the letter was about to be published.
"The Government's view is that we should remain in a reformed European Union and the civil service is able to support the Government in that role," he said.
"Of course, Members of Parliament, ministers, Cabinet ministers are able to make their own decision.
"But the Government is not holding back and hanging back from this. We have a full-throated view that we should put forward in front of the British people so they can make their choice."