The Serious Fraud Office's decision to drop its investigation into alleged bribery and corruption involving arms deals between BAE Systems and Saudi Arabia was overturned by the High Court today.

The ruling was an extraordinary victory for anti-bribery pressure group Corner House Research and the Campaign Against Arms Trade (CAAT).

The Serious Fraud Office (SFO) investigation arose out of BAE's £43 billion Al-Yamamah arms deal with Saudi Arabia in 1985, which provided Tornado and Hawk jets plus other military equipment.

In December 2006, the then attorney general, Lord Goldsmith, announced that the investigation into the arms company was to be discontinued.

Tony Blair, prime minister at the time, said that the Saudis had privately threatened to cut intelligence co-operation with Britain unless the inquiry was stopped.

Today Lord Justice Moses and Mr Justice Sullivan, sitting at the High Court in London, ruled that SFO director Robert Wardle "was required to satisfy the court that all that could reasonably be done had been done to resist the threat.

"He has failed to do so."