The commission said it had launched a "regulatory compliance case" that was looking at the connections between the Conservatives and Atlantic Bridge Education and Research Scheme.
Atlantic Bridge was founded in 1997 by Liam Fox, who is now the shadow defence secretary; its objectives include transatlantic communication and policy development between Britain and the US. The commission has received complaints that the charity is politically biased.
Its trustees include Fox, and its advisory board contains six members of the shadow cabinet, including shadow chancellor George Osborne, shadow foreign secretary William Hague and shadow home secretary Chris Grayling, as well as five American Republican senators and congressmen.
Atlantic Bridge declined to comment.