Bosses at more than a third of Britain's biggest businesses 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 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 said a Brexit would "deter investment and threaten jobs" but the total number falls short of the 80 or 50 that it had previously been suggested would sign.
The FTSE bosses make up just over a fifth of 198 business leaders, who, in a letter to The Times said: "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 but the chief executives of Heathrow and Gatwick have signed up.
Among the signatories are Tory donors and figures who have accepted government roles under Mr Cameron's premiership, according to The Times.
In another letter to The Times in support of Britain's EU membership, general secretary of the Trade Union Congress (TUC) Frances O'Grady said there was little positive for working people in Michael Gove's and Boris Johnson's vision for Britain outside Europe.
She said: "If the Brexit camp get their way, many vital workplace benefits that the EU has given us - paid holidays, extra maternity rights and better conditions for part-time workers - could be for the chop."
Former Labour business secretary and EU trade commissioner Peter Mandelson has said the Prime Minister's renegotiation deal should not be belittled as it was a "bridge" that sceptics could walk over back towards the EU.
In an opinion piece in The Guardian he said: "Whatever we may think about how David Cameron got to this point, this is a national fight we face now, not a party one, and for Britain's sake we need to make sure that the right side wins."
The support from some of Britain's most well-known businesses will be welcomed in Downing Street as a growing number of Conservatives declared they would be backing the leave campaign.
But Mr Cameron was forced to defend 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.
Leave.EU co-founder Richard Tice said: "We remember well how many large businesses and EU funded groups like the CBI said we should join the euro. How wrong they were.
"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.
"Brexit will reduce unnecessary regulatory burdens and cost on business, which can be used to invest in more jobs, not less."