The Confederation of British Industry (CBI) and CBRE polled 200 of the capital's business leaders, finding that three-quarters of businesses have customers – and almost two-thirds of firms have suppliers – in the European Union.
Ninety-three per cent of firms polled said London was a "good" or "very good" place to do business, and 83 per cent said London was a better place to do business than its global peers, including New York, Hong Kong, Paris and Tokyo.
Commenting on the findings, CBI London director Lucy Haynes said, "It’s clear that London is an international trailblazer".
"Keeping London ahead of our global competitors will be a top priority for the capital’s businesses," Haynes added. "Access to the European Single Market of 500m consumers is obviously a major boon to London’s firms, and this survey shows the strong desire of the capital’s firms to remain in a reformed European Union."
The CBI – which has come under attack from eurosceptic groups in the past for its pro-EU stance – said last weekend that it would be conducting a new national poll of its members' views of the UK's EU membership in response to Prime Minister David Cameron's reform deal.
"Most CBI members – though not all – have told us that being in a reformed EU is better for jobs, growth and prosperity," CBI director-general Carolyn Fairbairn said on Saturday. "With a final deal now in place, we will consult our members to ask for their views once again."
Separately, Martin Temple, who chairs the EEF, will tell manufacturing leaders at the industry group's annual dinner tonight that the campaign to leave the EU "has no tangible benefits to show voters and can only offer an abyss of uncertainty and risk".
"Our manufacturers, big and small depend on access to it, its supply chains and production networks," Temple will say of the Single Market. "Large and easy access to market matters far more than just its spending power. It matters because it is a platform for scale. Big domestic markets allow their companies to grow quickly and take a strong global position. Big in Europe, big in the world."
"In short, the great risk of leaving is that our country would be economically poorer," Temple will add. "Being in the EU gives us certainty, whereas those who argue we should leave can only offer uncertainty and risk with few, if any, real tangible benefits.
"The risk our companies might be less prosperous, the jobs of the people who work for them less secure, their future pension worth less. We have to convince people, in a language they understand that, economically, they and their families have a quality of life which is better for us being a member."
The EEF's most-recent poll of its members found that 61 per cent of manufacturers want the UK to remain in the EU, while just 5 per cent back Brexit. The remaining firms have yet to make up their mind, or said they will stay neutral in the run-up to the EU referendum on 23 June.