More than 200 entrepreneurs have thrown their weight behind the campaign for Britain to stay in the EU, warning that the economic shock of Brexit would be "hugely damaging" to the prospects of start-up businesses.
Among signatories to an open letter from Entrepreneurs for In are founders of companies that have rapidly become household names in the online economy, like Skype, Ebookers, Zoopla and Net a Porter, as well as some of the names behind brands including Innocent Drinks, Jack Wills, Yo! Sushi and Domino's Pizza.
And the Remain camp received a further boost as eight former US Treasury secretaries said in a joint letter to The Times that "a strong Britain inside the European Union" was "the best hope in our view for securing Britain's future, creating a more prosperous Europe and protecting a healthy and resilient global economy".
But a former governor of the Bank of England Lord (Mervyn) King said campaigners on both sides of the debate were exaggerating the potential impact of leaving or staying and cautioned against expectations of a "dramatic difference" from either result in the June 23 referendum.
"I think it's very important that people should not exaggerate the impact, either of staying in or of leaving," Lord King told the Bloomberg news agency, in comments which came two days after George Osborne released a Treasury analysis suggesting Brexit could shrink the economy by 6%.
"I do worry that people on both sides treating this as a public relations campaign rather than as a debate on the future of our country are inclined to exaggerate because they feel they are selling a position."
Larry Summers, who served as Treasury secretary in Bill Clinton's administration, told BBC Radio 4's Today programme that Brexit would be "the most isolationist deed in the last century" and would damage Britain's economic future and standing in the world, as well as reducing the effectiveness of the transatlantic special relationship.
David Cameron hailed the Treasury secretaries' intervention - a day ahead of the arrival of US president Barack Obama for a UK visit in which he is expected to caution against Brexit - as "important".
But prominent Leave campaigner and former defence secretary Liam Fox said some of the US signatories "go back a long way", and challenged their assessment that Brexit would diminish the UK's role in the world.
"There are many other areas than just money where Britain matters, security being one of them - we have the world's fifth biggest defence budget," Dr Fox told Today.
"Unless there is fundamental change in that European leadership we will have an imploding continent, and a British exit may give them the shock therapy they require to make the change necessary to stop Europe falling apart."
The Entrepreneurs for In letter, penned by Innocent Drinks founder Richard Reed - the vice-chairman of Britain Stronger in Europe - said that start-up companies benefited from "being able to do business within Europe's single market of 500 million consumers, with one set of regulations across 28 countries, and the ability to recruit the brightest people here and across Europe".
Reforms secured by Mr Cameron in his recent renegotiation of UK membership would "create even greater business opportunities for our firms", said the newly-launched group.
And they warned: "The economic shock of a vote to leave the EU would ... be hugely damaging to our businesses. Leaving could lead to lost investment, missed opportunities and lost jobs.
"The UK is currently the best place in Europe to launch and grow a business; leaving the European Union will undoubtedly undermine the ability of Britain's entrepreneurs to start-up, innovate, and grow. It is simply not worth the risk."
Among signatories were Zoopla chief executive Alex Chesterman, fashion designer Anya Hindmarch, Travelex founder Lloyd Dorfman, lastminute.com founder Martha Lane Fox, Skype founder Mattias Ljungman and Simon Woodroffe, the founder of Yo! Sushi.
The letter came as a former director general of the World Trade Organisation and ex-EU trade commissioner, Pascal Lamy, issued a warning over claims from pro-Brexit Justice Secretary Michael Gove that Britain would still be able to trade freely with the EU if it voted to leave.
"The notion that you exit the EU trade-wise with no price is simply a lie," Mr Lamy told BBC2's Newsnight.
"You export less, you produce less. You have less trade, less exports and less jobs. This is something that will never work. It's pie in the sky."
Meanwhile, the EU's most senior official admitted the 28-nation bloc had lost popularity because it over-regulates and interferes too much in people's lives.
Answering questions from British MPs in Strasbourg, European Commission president Jean-Claude Juncker said: "I think that one of the reasons that European citizens are stepping away from the European project is that we are interfering in too many domains of their private lives. And too many domains where the member states are better placed to take action and pass legislation."
Mr Juncker insisted the commission was "doing less" now in the face of such concerns.