The House of Lords upheld the decision of the Court of Appeal that the plaintiff's libel action against the defendants had rightly been struck out as an abuse of the process of the court.
The plaintiff had commenced libel proceedings against the defendants in respect of documents which had implicated him in a serious fraud with which he had never actually been charged. The documents had been disclosed by the SFO, as part of a file of unused material, to solicitors acting for one of the accused in the fraud case on whose behalf the plaintiff had been asked to give evidence, and the plaintiff had obtained them from the solicitors.
The action was struck out as an abuse of process, on the ground that the documents could not be used as the basis of a libel action without the leave of the court because their disclosure to the solicitors had been subject to an implied undertaking, similar to that which applied to documents produced on discovery in civil proceedings, that they would not be used for any purpose other than the defence of the accused concerned.
The Court of Appeal dismissed the plaintiff's appeal, holding that, although it was bound by the decision in Mahon v Rahn  3 All ER 687, namely that material disclosed by the prosecution to a defendant in criminal proceedings was not subject to any implied undertaking analogous to that in civil proceedings, the documents were nevertheless immune from suit because they had been brought into existence for the purposes of a criminal investigation.
Leolin Price QC and Julian Knowles (Jeffrey Green Russell) for the plaintiff; Andrew Caldecott QC and Catrin Evans (Treasury Solicitor) for the first and second defendants and (instructed by Crockers Oswald Hickson) for the third and fourth defendants.
Lord Hoffman said that the disclosure by the prosecution to the defence in criminal proceedings of documents as unused material in order to comply with its common law obligations did generate an implied undertaking not to use them for a collateral purpose.
The general principle was that the use of documents disclosed for the purpose of legal proceedings should remain under the control of the court, and the undertaking, which could always be varied in an appropriate case, was a necessary tool for preventing its process from being abused. Accordingly, the judge had been right to strike out the action for the reasons which he had given.
Persons taking part in a trial had absolute immunity from suit in respect of anything written or spoken in the course of the proceedings, and witnesses further had immunity in respect of statements made to a party and his legal advisers with a view to giving evidence. Since the policy of immunity was to enable people to speak freely, no rational principle could be identified which would confine the immunity for out of court statements to persons who were subsequently called as witnesses.
The same degree of necessity applied to the position of persons investigating the crime.
The correct test to be applied was whether the statement or conduct in respect of which immunity was sought could fairly be said to be part of the process of investigating a crime or a possible crime with a view to a prosecution or a possible prosecution in respect of the matter being investigated.