The High Court gave a ruling on Thursday that the Serious Fraud Office (SFO) acted unlawfully when it dropped a corruption inquiry into an arms deal involving BAE Systems and Saudi Arabia.
BAE Systems has been accused of giving bribes to Saudi officials in order to secure contracts for an arms deal worth £43 billion. BAE has denied any wrongdoing.
The SFO ended an investigation into the allegations in December 2006 on the grounds that it would harm national security by damaging relations with the Saudi's, who are key partners in combating terrorism.
Corner House and the Campaign Against Arms Trade (CATT) challenged the decision to end the investigation in the courts. The CATT said the dropping the probe broke the OECD Anti-Bribery Convention.