I was delighted to see that Michael Supperstone QC, a friend since my university days (1968-71), was recently appointed a High Court judge. But, despite what Downing Street believes, he was not a circuit judge before he joined the High Court.

The clue is to found in this statement from the Ministry of Justice three weeks ago. It refers to him correctly as "Mr Supperstone".

On Friday, Downing Street announced that the customary knighthood would be conferred on "His Honour Judge Michael Alan Supperstone QC". This is what he would have been called if he had been a circuit judge at the time.

But he was not. The clue is to be found in the biography issued as part of the Downing Street announcement. It does not say he was ever a circuit judge.

He was, in fact, a barrister at 11 King's Bench Walk, where the senior clerk, Philip Monham, is an old friend from my post-university, pre-journalism days (1972-74). He was a superb clerk even then.

It is not the first time that Downing Street has made this mistake. Indeed, they do it all the time. No doubt somebody copied an announcement made when a real circuit judge was promoted to the High Court, something that does indeed happen from time to time. Clearly, the Prime Minister's office just can't get the staff.

So here's a handy guide for Downing Street to cut out and keep. If the Ministry of Justice says that a circuit judge has been appointed to the High Court, you refer to him or her as "His Honour Judge ..." or "Her Honour Judge...". If the Ministry refers to the new judge as "Mr", "Mrs" or "Ms", they were not judges before being appointed. Part-time posts such as recorder and deputy High Court judge don't count.

There is one further complication. Downing Street has taken to delaying the announcement of the new judge's knighthood or DBE until after they have been in post for some weeks. By then, they will probably have acquired a judicial title and started sitting. So what Downing Street should have said was that a knighthood had been conferred on "the Hon Mr Justice Supperstone".

Got that? "The Hon" for High Court judges; "His/Her Hon" for circuit judges. No forename unless needed to avoid ambiguity. "QC", if appropriate, for circuit judges; the suffix is dropped when they become High Court judges.

Not very difficult, really. But if Downing Street can't get it right, how can anybody else be expected to?