Constitutional law expert Pierre de Vos on Tuesday apologised unreservedly for calling President Jacob Zuma a "gangster".

He made the statement after Zuma appointed Menzi Simelani as National Director of Public Prosecutions last week.

"In my criticism ? which I believe to be valid and based on the proven facts and a correct interpretation of the law and the Constitution ? I unfortunately reverted to the kind of intemperate language, which sadly has become all too common in our political discourse, by referring to our President as a 'gangster'," De Vos says on his blog.

"I regret using such intemperate language, which detracts from the substantive debate regarding the unfitness of Adv Menzi Simelane to hold office as the National Director of Public Prosecutions.

"I wish to apologise unreservedly to our President for the use of this intemperate language which, as the Presidency points out, does not contribute to the healthy and respectful debate so needed in our democracy.

Earlier, De Vos had said Zuma showed disregard for the Constitution and the law.

On Tuesday, the president's office issued a statement saying the expert had gone beyond the standards of decency and intelligent debate.