Goldman Sachs provided funds to the campaign to convince British voters to stay in the European Union, a source familiar with the issue told AFP Wednesday.

The source confirmed a Sky News story that said the Wall Street investment banking power donated "a substantial six-figure sum" in pounds to the anti-Brexit -- "British exit" campaign.

Sky News said the money went to the Britain Stronger in Europe group, which is fighting to convince people to vote in an upcoming referendum in favor of maintaining current ties with the EU.

British membership allows Goldman Sachs to work throughout the EU without having a banking license in each of its 28 member countries.

British Prime Minister David Cameron has demanded EU reforms which he deems essential to persuade Britons to vote to stay within the bloc.

The referendum is due by 2017, but British officials say it could be as early as this summer.

On Tuesday EU president Donald Tusk warned that a Brexit referendum "is more unpredictable than ever" and that an EU reform deal to persuade Britons to stay in the bloc would be difficult.