Senior prosecutors at the Serious Fraud Office have been accused of keeping secret key documents that detailed their misconduct in one of their biggest bribery investigations. Lisa Osofsky, the SFO director, and senior colleagues have been criticised for maintaining improper contact with a US private investigator during the investigation. The UK court of appeal heard on Wednesday that the SFO had to be forced to disclose significant documents that detailed the dealings Osofsky and her colleagues had had with the investigator. Adrian Darbishire, a QC for a British businessman seeking to overturn his corruption conviction, accused senior SFO prosecutors of a “deliberate and blanket decision” to withhold the documents. He alleged that Kevin Davis, the SFO’s chief investigator, intentionally deleted data on his mobile phone that logged his interactions with the Miami-based private investigator David Tinsley. Senior prosecutors had more than 40 calls or meetings with Tinsley that were not recorded, he claimed. Darbishire told Lord Justice Holroyde and two other judges that these contacts amounted to “malpractice” as the senior prosecutors were “party to an improper and unlawful attempt” to use Tinsley to persuade two defendants to plead guilty. The claims are likely to cause more discomfort for Osofsky, a dual US-UK national who took over as head of the SFO three years ago after working for the FBI and commercial firms. Last year a judge rebuked Osofsky and the SFO over “flattering” text messages she received from Tinsley, who was seeking to secure more favourable sentences for his clients. The judge, Martin Beddoe, said Osofsky and other senior SFO figures should not have had any contact with the investigator, who had no recognised legal role in the case. Beddoe criticised Osofsky for tacitly encouraging Tinsley, who had pressed the suspects to admit their involvement in a worldwide bribery scheme. The SFO has convicted four businessmen involved in a corrupt scheme run by a consultancy, Unaoil, that paid huge bribes around the world for years. One of the businessmen, Ziad Akle, has gone to the court of appeal to clear his name. Last year he was jailed for five years after being convicted of conspiracy to make corrupt payments to Iraqi officials. Tinsley had been hired by the Ahsani family, which owned and controlled Unaoil, to try to secure more lenient sentences for themselves. Two Ahsani brothers are due to be sentenced in a US court this year after admitting their guilt. Akle, a former Unaoil manager, is arguing that the SFO manipulated his trial by failing to disclose the full extent of its contacts with Tinsley, who founded the Miami-based private investigations firm 5 Stones. Darbishire alleged that “the actions of the SFO, in engaging Tinsley to approach defendants, in an attempt to persuade them to abandon not guilty pleas, behind the backs of their lawyers and during the trial process, amounted to an abuse of the process of the court”. Michael Brompton, QC for the SFO, told the court it accepted that its communications with Tinsley were “inappropriate” but they were not so egregious that they undermined the fairness of Akle’s conviction. He added Tinsley “had little or no influence over the SFO’s decision-making”, adding that the investigator had exaggerated the nature and extent of his contact with Osofsky to boost his credibility. The hearing continues.