Why did a UK judge place a reporting ban on Tommy Robinson's arrest?

Question

Tommy Robinson (A far right political activist in the UK) was arrested last week outside a UK courthouse.

As FOX Reported:

The judge in the case on Friday slapped a reporting ban on the case. The order bans reporters from reporting on a case if there is reason to believe the reporting could prejudice a trial. The order prevents reporting until the conclusion of the trial Robinson was reporting on.

The reasons for his arrest, are a very controversial topic, however, why would the judge initiate a reporting ban?

Answer 1

The block you quote gives a fairly clear summary of one possible reason.

The judge in the case on Friday slapped a reporting ban on the case. The order bans reporters from reporting on a case if there is reason to believe the reporting could prejudice a trial. The order prevents reporting until the conclusion of the trial Robinson was reporting on.

The law in England and Wales (and, I believe, the legal systems of the other constituent countries of the UK) allows the right to fair trial to take precedence over freedom of expression. One of the tenets of the legal system is that trials are meant to be decided based purely on the evidence presented within them, rather than on anything appearing outside. So, if a judge believes that media reporting on any external event has a possibility of influencing the decision making process, then said judge is within their rights to direct that the matter is not to be reported.

Answer 2

Since this question was first asked and answered, the reporting ban has been lifted, so there is information now available that wasn't before. The following is a summary of a BBC News article, and all the quotes are from that article.

Tommy Robinson was arrested, and has now been jailed, for contempt of court; specifically, for "potentially prejudicing a court case".

Specifically, he "was seen filming himself and people involved in the trial". He was then arrested for contempt on the grounds that:

His attempts to film defendants on that occasion could, the judge said, have "prejudiced" the jury, leading to an unfair verdict

Why did his contempt case initially have a reporting ban?

In this case, the judge ordered a temporary media black-out because he feared reporting Robinson's conviction could influence the jury in the very case Robinson was targeting.

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The article provides some background on this subject:

Contempt of court is the crime of ignoring the court and its constitutional role in making sure that justice is done. [...]

Why couldn't [journalists] initially report Robinson's arrest and jailing? Reporting restrictions are a long-standing part of the British legal system. [...]

This is not some new form of censorship directed at Robinson. These are rules that apply to us all, equally. If he is unsure about that, he's now got time on his hands to read a copy of Essential Law for Journalists.