Felicity Gerry: A prosecution can be made only if evidence is reliable

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The decision of the Crown Prosecution Service not to charge any police officer over the death of Ian Tomlinson at the G20 summit demonstrates the reality that a successful prosecution would follow only if there was reliable evidence on which any reasonable jury could convict.

In this case, consideration was given to charging assault and misconduct, but would have focused on the more serious allegation of manslaughter by way of the alleged unlawful acts of the police causing the death of Mr Tomlinson.

Two issues would have arisen at trial; whether there was an unlawful act, and if there was, whether that caused the death.

The burden of proving this is on the prosecution, and they succeed only if the evidence makes a jury sure of guilt.

The question of unlawful acts would depend on the evidence of witnesses (police and civilian) and footage, which in this case might have been conflicting. Such conflicts are easily highlighted by advocates in a criminal trial and often affect the ability of a jury to be sure of what happened.

Many cases involving death depend on expert medical evidence. Pathologists could not agree on the cause of death, and one expert is being investigated over separate matters. The CPS decision appears to have focused on the difficulties with medical evidence on the cause of Mr Tomlinson's death.

It is an expert's duty to help the court achieve justice by giving objective, unbiased opinion. An expert witness should state the facts upon which their opinion is based, give reasons and disclose any conflict of interest. Some have questioned the reliability of the experts who examined Mr Tomlinson. In criminal cases it is common for reliable experts to disagree.

The Court of Appeal has made it plain that if the outcome of a criminal trial depends exclusively (or almost exclusively) on disputed expert opinion then it may not be safe to proceed. Much depends on the issues in the case and what other evidence is available. In the context of crowd unrest and the consequence of police reaction, it would be possible for the advocates in a criminal trial to highlight conflicting evidence to such an extent that it would be impossible to ask a jury to be sure.

When a decision is made not to prosecute, those who feel aggrieved can seek a judicial review, which will succeed only if it was unreasonable, and that depends on an assessment of the evidence and the decision-making process.

Felicity Gerry is a criminal barrister