Generally speaking, private prosecutors face low risks in terms of costs and indeed can hope to recover some or all of their own costs, provided they are acting properly. If they act improperly, the situation changes completely.
In R (on the application of David Haigh) (Applicant) v City Of Westminster Magistrates' Court and others  EWHC 232 (Admin), the court was asked to quash orders for costs totalling over £230,000 which had been made against the claimant, a putative private prosecutor. The orders were under section 19 of the Prosecution of Offences Act 1985, in favour of three proposed defendants: the "interested parties".
The costs had been incurred by the interested parties after a District Judge had exercised her discretion to require them to be put on notice of the application for summonses, as a result of which they were involved at that preliminary stage. The costs were ordered by a different District Judge after the claimant sought to withdraw his applications for summonses (he had decided, at a late stage, and in the face of obvious difficulties with the private prosecution, to proceed via the conventional route of making a report to the authorities). Given that section 19 allows for an award of costs in favour of a "party to criminal proceedings", it must have been a considerable disappointment to Mr Haigh to be ordered to pay substantial costs to three people who in fact had never become parties (because the proceedings never launched).
The first point the court considered was whether there had been an "improper act". Although different views have been expressed over the years, the court proceeded on the basis that the measure of what is "improper" for the purposes of section 19 (costs between parties) is distinct from what is improper for the purposes of section 19A (wasted costs).
Beyond the issue of approach and the fact-specific arguments, the most interesting aspect of this case may be what it says about the risks taken in bringing ill-conceived private prosecutions.
There may be many reasons to bring a private prosecution: some laudable, others arguably less so. In many cases, the prosecutor is also the complainant and at least part of the motivation will be revenge (or something close to that) but that in itself does not make a prosecution improper, or render it liable to be stayed as an abuse of process. After all, the driving force behind many public prosecutions is the complainant, who is unlikely to have pure questions of public interest in mind. There is, though, a distinction between a prosecutor with mixed motives and one with a collateral aim, for example to obtain leverage in a civil action for damages. When the collateral aim is central to the prosecution, and central to the decision to mount it privately, the risks are higher. A private prosecutor has control of the case but if the reason for wanting control is to misuse the court process the consequences can be draconian.
In cases in which a private prosecution might be portrayed as an attempt to misuse the criminal process, usually for some other litigation aim, the stakes are raised. Legal costs may mount quickly on both sides, particularly when the background concerns a high value dispute. Mr Haigh's argument about proportionality in relation to the amount of costs to be ordered was firmly met by reference to equality of arms. The prosecutor paid handsomely for control, but ended up also paying comparable rates incurred by the interested parties in order to protect themselves.
Neither the District Judge nor the Administrative Court seems to have had much sympathy with the would-be prosecutor, or patience with his attempts to use the criminal courts in this way. The judgment emphasises that the right to bring a private prosecution is an important one and costs awards should not act as a deterrent, but it is clear that those principles do not amount to a license to misuse the system. This is not the first sign of unease at the thinking behind some private prosecutions, and is unlikely to be the last. The courts have said before that private prosecutions should be conducted with the "minister of justice" obligations of the prosecutor in mind: the court in Haigh added that there may be "more scope for scrutiny" in private cases. Where the prosecution cannot stand up to such scrutiny the risks are high.
In the end the court reduced the total orders, but only slightly: to £190,000. This was a costly venture into prosecuting, as well as a failed one.