Investment bank Goldman Sachs has reportedly given a "six figure" donation to the campaign to keep the UK in the European Union.
The US bank is said to have backed Britain Stronger In Europe, a cross-party group leading the In campaign.
Britain Stronger In Europe said it would not confirm the donation which Sky News said was a "substantial" sum.
It comes as David Cameron prepares to tell business to "make the case" for the staying in a "reformed EU".
In a speech at the World Economic Forum in Davos on Thursday, Mr Cameron will say the "voice of business" must be heard in the run-up to the referendum on the UK's membership of EU - which will take place before the end of 2017.
"If you agree with me about the changes I am arguing for in Europe… if you want a more competitive Europe, where the single market is completed, where there are more trade deals and fewer regulations… join me in making that case," he will say.
The exact timing of the referendum will hinge on the outcome of Mr Cameron's current renegotiation talks, due to come to a head at a summit of European leaders next month.
Goldman Sachs, which employs 6,000 people in the UK, has not commented on the donation. Although the sum being mooted is relatively small, the move would be symbolic and illustrative of the City's concerns about the implications of the UK leaving the EU.
A spokesman for Britain Stronger In Europe, which is chaired by ex-Marks & Spencer boss Lord Rose, told the BBC: "We are not going to comment on individual donations to the campaign at this stage.
"We will in time declare all our donations in line with standard electoral commission rules once we are the designated campaign. We are raising money from individuals and businesses in the UK from all walks of life. We believe that Britain is Stronger, safer and better off in Europe."
Leave.eu, one of the groups campaign for EU exit, dismissed Goldman Sachs' involvement.
Its founder, UKIP donor Arron Banks, tweeted: "No surprise to those of us who have said all along EU referendum will be a campaign of British people against the establishment of international bankers, multinational corporate tax dodgers and out-of-touch politicians."
Foreign Secretary Philip Hammond said the referendum date would only be decided once the UK has a deal on a renegotiated membership, amid speculation that an agreement next month would open the door for a vote in June.
Speaking in Edinburgh on Wednesday, he said: "If we get a deal, a June vote would be a possibility in terms of the timescales of the Referendum Act but that will only be decided once we've got a deal."
Meanwhile, Eurosceptic Labour MPs launched their own campaign to leave the EU, called Labour Leave, backed by the businessman John Mills.