A survey of the impact of withdrawal on London's services sector painted a bleak post-Brexit picture.
Even a Canadian-style free trade agreement would not prevent the downturn, according to the London First commissioned research by Frontier Economics.
London First's director of campaigns, Will Higham, warned that Brexit would come with a heavy price tag.
He said: "The services sector is a cornerstone of the UK economy. In fact, London is the leading European, and global hub for services.
"This brings enormous benefits to the UK as a whole. London combines leadership in financial services, creative industries, technology and business services. It is as though London is the New York, Los Angeles and Silicon Valley of Europe combined.
"Through our membership of the European Union, the UK reaps the benefits of increased competition and access to a much wider market, with fewer restrictions to this access.
"Critically, through EU membership, we have a voice at the table to shape this further. Leaving the EU would pose a massive risk to the UK services sector, and the wider jobs prospects for the next generation."
Former chancellor and Vote Leave supporter Lord Lamont said: "From day one the BSE (Britain Stronger in Europe) campaign have spent their entire time talking down the British economy and our ability to trade after a vote leave.
"It is disappointing but unsurprising that their message is so negative on the first official day of the campaign.
"The idea that leaving the EU would cause a 'disruption' to trade is little more than scaremongering. The big risk to the UK is remaining tied into a doomed political project that no-one wants, that takes power away from the British people and yet is ineffective in dealing with the very real immediate challenges it faces.
"A vote leave would be a signal that we are a proud independent country determined to trade across the globe, and to take back control of our borders and our democracy."