If the UK votes to leave it, the European Union project would collapse, according to Xavier Rolet, the head of the London Stock Exchange group.
"My own view is that if the UK were to leave, I don't think the EU would survive," he told CNBC Tuesday.
Rolet, who is a member of Prime Minister David Cameron's top-level business advisory group, has previously argued that a U.K. exit would be worse for some parts of Europe than the U.K.
"Europe is buried under so much debt and has structural issues," he said. "We see the Capital Markets Union (CMU) as not just the hope, but the last hope for the European project."
The CMU is an EU initiative designed to made a single market for capital within the trading bloc, which it is hoped will make it easier for businesses to raise finance and grow. Many of the continent's biggest businesses were founded around the turn of the 20th century rather than the 21st, and some blame the funding system for this dearth of new, successful companies.
Europe needs a fundamental re-shaping of how it grows and develops businesses, with a re-focus on small businesses rather than national champion blue-chips, Rolet argued. Of the 100 most successful global start-ups since 1975, only one (Inditex, the owner of Zara) is from Europe, while 26 are from California, according to The Economist. As it is unlikely to compete with faster-growing economies on the cost of its workforce, the continent may need a greater focus on new job creation via new ideas and new businesses.
"There is no other solution to the unemployment and growth problem in Europe but SMEs. New jobs are not going to come out of the blue chip sector," Rolet told CNBC.
"If we cannot create good-paying jobs which can only be created in a capitalist economy by innovation, we will not get that growth."
He also praised the financing environment in the U.S. compared to Europe.
"Fundamentally, America is an upside-, growth-, forward-looking society, and we believe this is linked to U.S. companies using risk capital, equity in particular, as a funding vehicle. Europe is obsessed with debt. It is a downside instrument, focused on default, on servicing the debt, on loan guarantees and by definition Europe has a more negative outlook," he added.