Maintaining London's access to skilled EU workers after Brexit is "vital" for the capital's future prosperity, the report by 'London First' said.
It said business leaders were worried any restriction on skilled migration would "impede London's ability to attract talent".
London First describes itself as a non-profit organisation with a mission "to make London the best city in the world to do business".
Prime Minister Theresa May has not confirmed her plans for freedom of movement after Brexit.
But she has previously promised to secure a settlement that "addresses the concerns of the British people about free movement".
The report, which is based upon talks with business leaders, found that leaving the EU will present "challenges" and "opportunities" to the capital to keep it competitive on the world stage.
In the report, London First said the city had "one of the greatest concentrations of talent and skills in the world" and future policies should attempt to maintain this.
The report did find that leaving the EU could provide access to new markets and sources of investment for businesses in the city.
But it said "challenges" created by economic growth would have to be addressed to keep the city "open for business".
These included better transport infrastructure, improved skills training in the capital and the faster delivery of new housing.
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