Crown Prosecutors have to ask themselves two questions when deciding whether or not a case should go ahead.

The first test is: 'Is there enough evidence against the defendant?' This objective test means that a jury or magistrates, properly directed, are more likely than not to convict the defendant of the alleged charge.

If there is not a realistic prospect of conviction the case must not go ahead, no matter how important or serious it may be.

If there is a chance, the second test then comes into force - 'Is it in the public interest for the CPS to bring the case to court?'

Before making a decision under this test the Crown Prosecutors must consider public interest and balance factors for and against prosecution. The public interest factors vary from case to case. According to the CPS the more serious the alleged offence the more likely it will be a prosecution is needed in the public interest.

But a prosecution is less likely if a court would be likely to fix a minimal or token penalty, or the loss or harm connected with the offence was minor and the result of a single incident.

These explanations have been given to two 13-year-old girls - one the alleged victim of assault in a sexual way and harassment, and the other an alleged victim of harassment - both by the same man.

The senior Crown Prosecutor who applied the tests and decided not to go ahead with the police charges says in a letter to the girls: 'I appreciate you may be disappointed with the outcome of this case' - adding they may wish to show the letter to their parents. Too right they are disappointed.

The girls bravely went through the trauma of giving statements, followed up with statements on video, and along with their families had high hopes of a conviction.

Now, more than six months later, they have been given the news that the case has been dropped - or to use official parlance 'proceedings are at an end' - with no possibility of a change of mind.

Assuming a correct decision has been made it goes beyond the realms of common decency that the girls have had to wait so long for a decision.

Surely it is a matter of public interest in a case such as this that justice - or the lack of it - is swift.

Is it any wonder the parents and families of the girls are angry and distressed, to the point of believing they made a mistake going to the police and trusting British justice?

In one of the letters the CPS says the man accused says he was only having a bit of fun when he asked one of the girls if she was interested in modelling for him to take photographs to send over the Internet to Zimbabwe. The more she showed, the more money she would get.

But no specific arrangement was made and no photographs were in fact taken. Therefore, what took place is not enough in law to make out the offence charged, says the CPS. There's no prizes for guessing what a jury of ordinary right minded people would have made of that, had it gone to court.

Of all the people the law should protect, children must be at the top of the list. The message this decision sends out to would-be perverts is unthinkable.