The Metropolitan Police has been criticised over its decision not to promote Asian officer Ali Dizaei.
Ch Supt Ali Dizaei's book has been serialised in a newspaper
Ch Supt Dizaei, one of 10 officers competing for the rank of commander, wrote a book critical of Scotland Yard.
The Metropolitan Black Police Association (MBPA) said it suspected the decision "was not free from bias".
But the Metropolitan Police Association (MPA), said: "Their allegation of bias amounts to a charge of discrimination. That is wholly unsubstantiated."
Last week Scotland Yard expressed "regret" that Mr Dizaei had chosen to publish his book, Not One Of Us.
It is highly critical of a four-year, multi-million-pound corruption investigation launched against him in 1999.
Mr Dizaei, now borough commander in Hounslow, west London, told BBC London that he would "just have to wait and see" what reasons were given for his lack of promotion.
"All I know is that I worked very hard in the last two years," he said. "My own line managers graded my performance as 'excellent' for the last two years."
Mark Simmons and Stuart Osborne, currently temporary commanders with the Met, and Ch Supt Ian Quinton of Norfolk Constabulary were named the new commanders on Wednesday.
The MBPA said it was "extremely disappointed" that Mr Dizaei was not promoted.
The National Black Police Association (NBPA) questioned whether Scotland Yard had a "vendetta" against Mr Dizaei.
NBPA President Keith Jarrett said he would seek an urgent meeting with Met Commissioner Sir Ian Blair to get "answers" as to why Mr Dizaei's application had been unsuccessful.
But MPA chair Len Duvall said: "We categorically deny the Metropolitan Black Police Association's unwarranted attack on the integrity of our promotion process."
He added: "We believe we have chosen the best people to join the highest echelons of the police service.
"We will expect them to use their skills, expertise and professionalism to deliver for the people of London."