Allegations that China has sought to spy on Members of Parliament has caused understandable concern in Parliament and beyond, but the collapse of the criminal case against the individuals said to be involved has caused even more concern.

It is worth recalling what had happened in the lead-up to the decision of the Crown Prosecution Service (CPS), which decides whether to pursue prosecutions in criminal law cases, to drop its case against these two individuals.

They had been arrested and charged, and the case was listed for trial. In order to get that far, the CPS had to have concluded both that there was enough evidence against the defendants to provide a realistic prospect that a jury would convict them at trial, and that it was in the public interest to prosecute them.

The second criterion – that it was in the public interest to prosecute – has not been in doubt and one can see why.

It is clearly in the public interest to prosecute someone spying on Parliamentarians for a foreign power.

The CPS decision to drop the case was based on their changing view of the first criterion – whether there is sufficient evidence to provide a reasonable prospect of conviction, and it is worth exploring that in some detail.

First, it is important to say that the CPS have a clear duty to keep that criterion under review and, if the evidence they can present to a jury no longer constitutes a case which can meet the required standard, they should not proceed.

It is also a clear feature of our criminal justice system that it is the CPS alone which makes that decision. It has been suggested that someone in the Government, worried that this prosecution would damage diplomatic relations with China, sought to persuade the CPS to change its assessment of the evidence it had and to drop the case.

I think that is a red herring. My view as a former criminal lawyer and Attorney General is that something else has happened here. I do not think the Government killed this case, but I do think the Government allowed it to die.

Let me explain.

It is now clear that the failure of this case comes down to one specific gap in the evidence which the CPS came to believe it needed in order to continue its prosecution in this case. It is also clear that if that gap could have been filled, the case could have proceeded, leaving a jury to decide whether spying offences had in fact been committed.

That gap in the evidence relates to one specific component in the offence charged which the Prosecution must prove to a jury – namely that the material being passed on by those accused would have been useful to 'an enemy' of the United Kingdom.

The CPS maintain that the only country to whom this information would have been useful was China and therefore that it was necessary to establish that China was, at the relevant time, an enemy of the UK.

They had previously believed that this could be established by the evidence they had, or expected to get, from the Government, but eventually concluded that it could not be. That is what caused the failure of this case, so we need to understand what caused the CPS to change its mind.

Between original charge and the case coming to court, the Court of Appeal had given a judgment in a different case on what the term 'enemy' meant in the context of charges under the Official Secrets Act 1911, the Act under which the current case was charged.

The Court of Appeal thought that, ultimately, a jury could decide for itself what the term meant, but that it would certainly include a country which presented a current threat to our national security.

The CPS interpreted that to mean that they would then need to present evidence to a jury that China was indeed a current threat to our national security, at the time at which the activities of the

Defendants were being carried out.

I should say at this point that there is some disagreement among those who are analysing events in this case that the CPS were right to conclude that it then had to prove China was a current national security threat in order to proceed or, if it did, that it did not already have the evidence to do so, but I will spare you the legal arguments on that for now.

For the purposes of this discussion, what matters is that the CPS, advised by senior barristers, concluded that they needed to prove that point and that they needed more evidence to do so.

The CPS also concluded that the further evidence they needed could only come from the Government, and the Government's witness in the case was the Deputy National Security Adviser (DNSA).

As you may have seen, he ended up making three different witness statements in the case, the latter two in response to repeated CPS requests for clearer and more definitive statements on the key point of whether China was a current national security threat to the UK at the relevant time.

The reason the CPS changed its mind about proceeding was that the DNSA did not provide that additional evidence to the satisfaction of the CPS, and so the case collapsed. The crucial question therefore is why did the Government not provide the extra evidence asked for? There must be three possible answers:

1. The further evidence was not given because it did not exist;

2. It was not given because the Government refused to give it, believing that the CPS already had all it needed;

3. It was not given because the Government was in fact content for the case not to proceed.

Let's consider each possibility. On the first, the CPS clearly believed that evidence of China posing a current national security threat existed, or they would not have kept asking for it, and would likely have abandoned the case earlier.

This was surely a reasonable belief, as public statements by Ministers and Intelligence Agency Heads from the relevant period about Chinese espionage and other activity could certainly have been used to support the contention that China was such a threat.

It does not appear to be necessary to exclude the possibility that, in addition to being a national security threat, China also constituted an economic opportunity or even a state with which we must engage on other matters such as climate change, only that, among other things perhaps, China also represented a current threat to our national security.

I think a jury could be expected to conclude that countries are often more than one thing to each other but that the relevant question was on national security.

On the second, Government Ministers have made it clear repeatedly that decisions in this criminal case were for the CPS to take, and this must be right.

It follows that judgments on the sufficiency of evidence were also for the CPS alone to make and if the CPS told Government that more evidence was needed in order to proceed with the case, that judgment had to be have been accepted, however reluctantly.

It was only for the Government to decide whether or not to provide the extra evidence asked for. It would also have been very obvious to the Government, on the CPS account of events, that the case would not proceed without the extra evidence asked for and so, whether the Government believed the CPS were making the right legal judgment or not on the necessity of further evidence, it certainly knew what the consequences of failing to provide it would be.

Government Ministers have also said repeatedly that the Government was extremely disappointed that this case did not proceed. Someone who is disappointed that something has not happened clearly wanted it to happen and, logically, could be expected to do all they could to make it happen. Again, the Government knew what the CPS needed it to say, and knew the case would collapse if it did not. Evidentially speaking then, how much more was needed?

Not much, I would suggest. In his first witness statement, the DNSA said that Chinese Intelligence Services 'conduct large scale espionage operations against the UK and other international partners to advance the Chinese state's interests and harm the interests and security of the UK'. Sounds like a national security threat to me, and would saying so in terms really be a substantive departure from that evidence?

In his second statement, the DNSA said that China 'also presents the biggest state-based threat to the UK's economic security'. Isn't economic security national security?

In his third statement, the DNSA said that, during the relevant period, 'the UK Government publicly articulated a number of concerns about the long-term strategic challenge that China poses to the UK under the Chinese authorities, including the increasing Chinese espionage threat posed to the UK'.

Again, surely it would only need a half-step more to say in terms what the CPS wanted said in terms, but we know there was no further statement after that, despite a high-level Government meeting taking place after it had been submitted to decide what, if anything, more the Government would do. I repeat, by that point the Government knew if it did not take that further half-step, the case would collapse.

You could certainly argue that the CPS might have tried their luck with a jury on what they already had because it wasn't far away from what they ideally wanted, but by the same token it wouldn't have taken much more from the Government to get to what the CPS ideally wanted, yet Government chose not to provide it and allowed the case to fail.

Which brings us to the third possible explanation for what happened, an explanation the Government fiercely denies but, when you consider the alternatives, must be the most likely.

If the Government had wished to stop this case from collapsing, it could have done so. It knew what was required. What was required was very similar in substance to evidence it had already been prepared to give. It knew that the CPS would drop the case if that further evidence was not forthcoming. In addition, it must have been clear what the Prosecution case would be.

Adding text to refer to a positive economic relationship with China, which the DNSA did in his later statements, was hard to understand. Although it may be true, it was not relevant or necessary and could only dilute that case.

It is hard not to conclude that those in Government who wished to avoid the awkwardness of this case for diplomatic relations with the Chinese decided to take advantage of the moment and decline to intervene to save the case.

It would not be necessary to interfere with CPS decision-making or seek to change evidence already submitted, just for Government to sit on its hands and allow the case to run into the buffers, preparing to say, as was at least partly true, that it was all because of a CPS decision.

I think it was decided that this was a good way out of a potential diplomatic crisis – presented, perhaps unexpectedly, by excessive caution on the part of the CPS about the sufficiency of the evidence they had. Could the CPS have avoided the collapse of this case by proceeding regardless? I suspect they could.

Could the Government have prevented its collapse if they had chosen to? I think it could have. It decided not to, and the consequences of that decision are serious. Whether the Defendants on this case were guilty would have been for a jury to decide, but their acquittal sends a clear signal to others who may be tempted to spy against their country – you might just get away with it.