British-Canadian national Dahdaleh has pleaded not guilty to eight charges relating to events between 1998 and 2006 at Alba, the world's fourth-largest aluminium smelter, which is majority-owned by the Bahraini state.

Jawad, one of five Bahraini deputy prime ministers, could not be reached for comment on the letter, which was read to the court by Dahdaleh's lawyer Nicholas Purnell on Thursday. The prosecution did not dispute the authenticity of the letter.

"At the request of Allen & Overy [the London law firm then acting for Dahdaleh], we hereby confirm that the board of directors of Aluminium Bahrain knew of and approved all contracts entered into by Alba, including knowing of and approving payments made by Victor Dahdaleh," the letter said. "This was entirely in accordance with Alba practice."

Those assertions are relevant to Dahdaleh's defence, which is that he had "principal's consent" for the payments he made to then Alba chairman Sheikh Isa bin Ali al-Khalifa and to then chief executive Bruce Hall.

Dahdaleh is charged under an old anti-bribery law from 1906 which says that payments are not corrupt if they are made by an "agent" on behalf of a "principal" who consented.

The SFO's position is that the rightful principal was the board of Alba and that it never approved the payments.

Isa, who is named as a co-conspirator in the indictment but is not taking part in the trial, has denied any wrongdoing in a statement issued by a Paris-based lawyer.

Hall has pleaded guilty to a charge of conspiracy to corrupt and accepted that he was part of a criminal conspiracy with Isa and Dahdaleh. He is co-operating with the SFO.

It is not the first time an SFO investigation into alleged corruption in the region has come under political pressure.

In 2006, a probe of an arms deal between British defence group BAE Systems and Saudi Arabia was dropped after the British government told the SFO it could hurt national security.