The appointment of Seumas Milne as Labour's strategy and communications chief highlights a lack of "professionalism" by Jeremy Corbyn, Lord Mandelson has claimed.
The Guardian’s associate editor is to join the new leader’s team "on leave" from the newspaper but some moderate MPs have expressed fears he could push the party further to the left.
But Lord Mandelson told BBC's Week in Westminster that the appointment showed that Corbyn lacks “professionalism”, according to the Press Association.
The former cabinet minister said: "I don't think he's growing into the job at all, no.
"I don't think he is showing any professionalism in his leadership of the Labour Party and you see from his appointment of his strategy and communications director Seamus Milne, whom I happen to know and like as it happens but (is) completely unsuited to such a job.
"He has little connection with mainstream politics or mainstream media in the country and yet he's in charge of communications for the Labour Party.
"That doesn't sound very professional to me."
Milne has caused controversy in the past with comments over the murder of Lee Rigby, the attacks on 9/11 and the Iraq war.
In the Telegraph, Tom Harris, a former Labour MP, said of Milne: “In this appointment, Corbyn has stuck two fingers up at his detractors. ‘You think I’m going to compromise my views just to be popular? Just to be elected? Well, look who I’ve just hired!’”
He described the columnist as a “hate figure for the right of the party” and said that some Labour voters would be “appalled” at some of Milne’s views.
Lord Mandelson, who was a driving force behind reforms to the party in the 1980s and 1990s and at the heart of New Labour's spin operation, has faced calls for his expulsion from the party for "openly inciting insurrection" against Corbyn.
He, however, insists he is going nowhere.