Maxwell, the late owner of Mirror Newspapers, invested $2 million in Mr Romney’s first private equity fund, which launched the controversial career in finance that the Republican presidential challenger now cites as proof of his ability to lead the US to prosperity.

He was recruited by Lyons, a late colleague of Mr Romney’s at Bain & Company and one of the "Guinness Four" who were convicted in 1990 over the infamous share-trading fraud at the drinks firm. Lyons and his family invested almost $3 million in Mr Romney’s fund.

Both Lyons and Maxwell kept their money in tax havens. The discovery of their financial links to Mr Romney comes amid mounting pressure on the former Massachusetts governor to disclose details of his own offshore holdings, including a Swiss bank account.