George Bush's grandfather, the late US senator Prescott Bush, was a director and shareholder of companies that profited from their involvement with the financial backers of Nazi Germany.

The Guardian has obtained confirmation from newly discovered files in the US National Archives that a firm of which Prescott Bush was a director was involved with the financial architects of Nazism.

His business dealings, which continued until his company's assets were seized in 1942 under the Trading with the Enemy Act, has led more than 60 years later to a civil action for damages being brought in Germany against the Bush family by two former slave labourers at Auschwitz and to a hum of pre-election controversy.

The evidence has also prompted one former US Nazi war crimes prosecutor to argue that the late senator's action should have been grounds for prosecution for giving aid and comfort to the enemy.

The debate over Prescott Bush's behaviour has been bubbling under the surface for some time.