Joining industry groups from various Commonwealth countries, CII highlighted the "border-free" access to Europe for Indian firms in the UK as a strong factor in the UK's attractiveness to Indian investment.
"India invests more in the UK than in the rest of Europe combined, emerging as the UK's third largest FDI investor. Access to European markets is therefore a key driver for Indian companies coming to the UK," CII director-general Chandrajit Banerjee said in a statement.
His views were echoed by industry bodies from Canada, South Africa, Kenya and Jamaica.
The UK's own Confederation of British Industry made its stand in favour of staying in as the "best of both worlds". Its Director-General Carolyn Fairbairn said: "The UK has the best of both worlds by being able to trade easily with nations both from the Commonwealth and from the EU. Our membership of the EU gives us a home market of 500 million customers, while EU trade deals open up new opportunities in markets across the world, including our historic Commonwealth friends.
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She warned that leaving the EU would mean going back to the drawing board. "We would have to undertake lengthy negotiations to secure brand new deals, which we know from experience can take many years. Having a strong British voice in the EU is vital to the interests of the Commonwealth."
Khanyisile Kweyama, chief executive of Business Unity South Africa, said: "We want the UK to remain in the EU, to build a more prosperous Commonwealth, Africa and Europe."
John Manley, president and chief executive of the Business Council of Canada, added: "Canadian companies invest more in the UK than anywhere else in the world except for the US, and they do so in large measure because they consider the UK a gateway to the rest of Europe."