The trial of the six climate-change activists accused of plotting to shut down Ratcliffe-on-Soar power station in Nottinghamshire has collapsed, as most readers of this site will no doubt already know. The mainstream media has focused almost entirely on the role of undercover police officer Mark Kennedy, and the implication is that it is his role that caused the case to collapse.

Whilst his activities are at best dubious and at worst amount to entrapment and incitement, it seems to me that they are masking the main story. The officer recorded most of what happened - in undercover cases the undercover officer rarely gives evidence in person, and the recordings are used instead. So the fact that he didn't want to give evidence for the prosecution would not have caused the case to fold. The CPS have now stated that the reason they dropped the case against the six activists was not because of Mark Kennedy at all, but because "police officers have suppressed vital evidence".

This has worrying implications for other activists already convicted as part of this operation in earlier trials. The cost to the taxpayer of abandoning the current trial is thought to be around £27,000. I have no idea how much the previous trials cost, but if this suppressed evidence is that crucial, it is likely to result in a retrial which will cost a fortune.

Now, it is not totally out of the ordinary for the police to hold back certain important information. Sadly I see it happen all too frequently in trials I deal with. However - it is usually information that, although potentially of assistance to the defence, is hardly earth-shattering. It is usually a copy of a document that shows a minor discrepancy between an initial police report and a later statement, or the details of a witness who doesn't assist the prosecution (but usually doesn't assist the defence either as they didn't see anything relevant).

Although the defence are entitled to see such material and it is sometimes overlooked, it is usually discovered shortly before the trial or on the first day of trial by the prosecutor, who will come clean immediately, apologise, and disclose it. In most cases it doesn't cause any real delay as it is of no use to the defence - and so we crack on.

Occasionally it is a useful witness or a document with a major discrepancy that really helps the defence, in which case an adjournment may be needed for the defence to chase that information and get the relevant witness to court. Occasionally I feel the police have deliberately held it back, but more often than not it is because the officer hadn't appreciated its potential use to the defence and so did not realise it should be disclosed.

In very rare cases the evidence will be so compelling and so powerful in favour of the defence, or so strongly undermines the prosecution, that the whole case is dropped there and then. This is what has happened with the Nottinghamshire case. Some evidence (which is not yet public knowledge) that the police have known about all along was not disclosed to the defence; the prosecution lawyers stumbled upon it in the last few days leading up to the trial as they finalised their preparations, and decided to drop all charges against all defendants.

In my experience this must be very damning evidence, supporting the contention that the activists were innocent of any illegal activity (let's not forget - planning a protest itself is not illegal and nor is trespassing - trespass is not a criminal offence) in order for the prosecution to throw the towel in immediately on all charges against all parties.

If it is that strong the police must have known it pointed to the innocence of the defendants, and by "suppressing" it they have done something morally wrong and downright criminal (if indeed it was deliberately suppressed).

Now, one might argue that the police must know such evidence shouldn't be suppressed and evidence that points to the innocence of the defendants should be disclosed as a matter of common sense, but the sad fact is many police officers are poorly trained as to the law.

The law states that the prosecution must disclose to the defence "any material which is likely to undermine the prosecution case or assist the defence". That is quite a low threshold. It doesn't have to definitely undermine or assist - but if it appears likely to, it must be disclosed.

The problem is that the law, as it stands, puts the decision entirely in the hands of a police officer. Every case has a 'Disclosure Officer' appointed, a police officer - in most cases, the arresting or investigating officer - who in my experience is almost always convinced the defendant is guilty.

In an ideal world every single bit of evidence would be disclosed to the defence and if there was anything useful to our case we would be able to use it. However that is not the way it works. The prosecution have the discretion, but instead of a trained CPS lawyer making that decision, it is done by a police officer.

It is a simple fact that CPS lawyers are way more objective and fair in their approach to prosecuting cases than most police officers. If they think a case is rubbish they will drop it. If they see documents that will assist the defence they disclose them - but most importantly they know the law well and so understand the evidential rules and the relevance of certain pieces of evidence. If they were the ones who decided what was disclosed this issue with suppressed evidence would happen far less. Prosecutors are penalised if cases get dropped at the last minute, on the basis they should not prosecute cases unless there is a good prospect of conviction. So if they see evidence that undermines the case, they are usually keen to disclose it and, if appropriate, stop the case before any more time and money is wasted preparing it.

Sadly it is left with the police, who more often than not are gunning for a conviction, and are in some cases rather reluctant to assist the defence. The police can often forget that their duty is to investigate the offence not the suspect. If they find evidence that points to the conclusion that the suspect is not guilty, they often do not follow it up.

Routinely I have clients who give alibi evidence in interview. Instead of the police (as they used to) going to get a statement from the alibi witness whilst the suspect is still in custody (which means the suspect and alibi witness get no chance to collude) they tend not to bother and to charge the suspect and leave it to the defence to get the statement afterwards. Obviously an alibi witness who gives evidence at trial is subject to cross-examination from a prosecutor - usually along the lines of "You've made this up to support the defendant. You and he/she have colluded and come up with this account".

Had the police checked the alibi at the time they would have been able to very quickly establish whether the suspect is lying or (probably) telling the truth.

I am saddened to say I have had a case myself where a police officer deliberately failed to disclose mobile phone records that proved my client was telling the truth and the alleged victim of a Harassment case had been lying. The "victim" who made the false complaint was not prosecuted and the policeman was not either. Had the prosecutor not found out about it (on the day of trial) and dropped the case, then the trial may have gone ahead - just the defendant's word against the "victim" and he may well have been convicted.

In that case a police officer deliberately suppressed evidence that proved my client's innocence. Had it remained hidden my client (a man who had never been in trouble before) may well have been found guilty and imprisoned.

So, was the officer prosecuted for attempting to pervert the course of justice? No - he said he didn't realise it was relevant evidence (despite the fact that we had requested "any telephone transcript evidence" three times) and that he made a mistake when he said to the CPS there was "no phone evidence". He said he meant there was no phone evidence that would help the prosecution, so he thought it was okay to say there was none at all. He got a telling off from his sergeant and no formal disciplinary action. He continues to be a disclosure officer on numerous cases.