Leaving the European Union would threaten jobs and put the UK's economy at risk, business leaders from some of Britain's biggest companies have said.
Bosses — including those of BT, Marks & Spencer and Vodafone — signed a letter published in the Times, saying an EU exit would deter investment in the UK.
However, Leave.EU said leaving would cut "unnecessary" regulation and costs.
On Monday, PM David Cameron told MPs leaving the union would hurt working people in the UK "for years to come".
Chairmen or chief executives of 36 FTSE 100 companies signed the letter backing the campaign to remain in the EU.
The FTSE bosses were among a total of 198 signatories from the business world, including the chief executives of Heathrow and Gatwick airports.
The letter said the PM had secured a commitment from the EU "to reduce the burden of regulation, deepen the single market and to sign-off crucial international trade deals".
They wrote: "Business needs unrestricted access to the European market of 500 million people in order to continue to grow, invest and create jobs."
"Britain will be strong, safer and better off remaining a member of the EU," they added.
'Bullying'
Richard Tice, co-founder of Leave.EU — which is campaigning to leave — said Downing Street had admitted using taxpayers' money and "applying pressure" on FTSE chairmen and chief executives to sign the letter.
"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," he added.
BBC business editor Kamal Ahmed said two leaders of US firm Goldman Sachs in Europe had also signed the letter, which he said signalled "wider concern among major American banks based in London about the risks of leaving the EU".
But he said Brexit campaigners claimed smaller businesses were "much more sceptical" about the advantages of staying in the EU.
He added that there were some notable absences from the letter — such as bosses of FTSE 100 firms including Tesco, RBS and Barclays.
Meanwhile, Labour's former home secretary Alan Johnson is due to warn that leaving the EU could put up to 50,000 UK manufacturing apprenticeships at risk.
In a speech later, he will outline Labour's case for remaining in the 28-state union, saying two thirds of British jobs in manufacturing depend on demand from Europe.
Senior figures from both sides of the debate will also continue to campaign ahead of the vote — due to be held on 23 June.
Mr Cameron will visit a firm in Berkshire, while the chairman of Vote Leave, Lord Lawson, is due to make a speech at Chatham House.
'Into the dark'
Divisions in the Conservative Party over the referendum were laid bare during a debate in Parliament on Monday.
More than 100 Conservative MPs want to leave the EU — including five full cabinet ministers and Mayor of London Boris Johnson.
Putting his case for staying in the EU to MPs, Mr Cameron said the choice was between an "even greater Britain", or a "leap into the dark".
However, a succession of Tory MPs questioned the substance of the PM's agreement with EU leaders — announced on Friday.
Mr Johnson asked how the deal negotiated would "in any way" return sovereignty to the UK.
The prime minister defended the deal — outlined in a new government document — telling MPs it would give the UK a "special status" within the EU and ensure it never became part of a European super-state.
But Tory backbencher Jacob Rees-Mogg said the EU was a "failed" body and the UK should "make our own path".
Labour leader Jeremy Corbyn said his party could make a "real" argument for the benefits of EU membership.
According to BBC research, 142 Tory MPs will campaign to remain in the EU, 120 to leave, while 68 have yet to make their positions clear.
The overwhelming majority of Labour MPs back continued EU membership as do the SNP, Plaid Cymru and the Liberal Democrats but the Democratic Unionists and UKIP are opposed.