Justice Stephen Breyer, writing a unanimous judgment for the court, agreed with the investors.
However, the supreme court has now thrown out his appeal in a unanimous judgment by five senior law lords.
The Court should have respected the nearly unanimous judgment of Congress that the law would protect the electoral gains of minorities and prevent backsliding.
In a statement, the ANC said it respected the unanimous judgment by the constitutional court, declaring the party had "full confidence in the judiciary".
"The conviction, placed in the light of the fresh evidence, constitutes a miscarriage of justice and must be quashed," the court said in a unanimous judgment.
However, the unanimous judgment delivered by three of the most senior judges in England and Wales will make it difficult for government lawyers to overturn the ruling in the supreme court and avoid delay.
In a unanimous judgment, the 11 constitutional court justices said a damning 2014 report by the public protector, Thuli Madonsela, into the spending was legally binding and that remedial action should have been upheld.
This year Johnson Matthey, the bullion and chemicals company, won a unanimous judgment at the court of appeal in London dismissing an earlier claim on behalf of five workers.
