The appointment of a left-wing columnist as Labour's strategy and communications chief highlights a lack of "professionalism" by Jeremy Corbyn, Lord Mandelson has said.
Guardian associate editor Seumas Milne is joining the new leader's senior team "on leave" from the newspaper, to the anger of some moderate MPs amid fears it will push the party further to the left.
Lord Mandelson - a driving force behind reforms to the party in the 1980s and 1990s and at the heart of New Labour's spin operation - also told BBC's Week in Westminster he was unimpressed by Mr Corbyn's time at the helm since his surprise election.
"I don't think he's growing into the job at all, no," the former cabinet minister said.
"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."
The arch-Blairite has faced calls for his expulsion from the party for "openly inciting insurrection" against Mr Corbyn, but he has insisted he is "not going anywhere".