Scotland Yard's Complaints Investigation Bureau is investigating allegedly racist comments he made on television on 28 March when he said: 'Some members of the black community pose the greatest threat to law and order.'
Last week Sgt Bennett was unrepentant: 'I was talking about Yardies, armed killers. The vast majority of black, Asian or white people are perfectly nice. I'm not a racist. Three members of my family are black - that is not well known. But the police are never going to see eye to eye with the sort of people I was talking about, regardless of their colour.'
The investigation has annoyed some colleagues in the Met, who suggest it is a warning shot at Bennett as the Metropolitan Police's image comes under ever tighter control from the Commissioner, Paul Condon. They claim that the CIB is too busy to deal with deaths in custody yet has time to investigate this case.
The Police Federation is the nearest thing the police have to a trade union, and its officers have traditionally stuck to issues such as pay and disciplinary cases. But since becoming the Met Federation's chairman, Sgt Bennett has become one of the best-known policemen in the country and an outspoken critic of police and criminal-justice reforms - often voicing views that other, more politic individuals are wary of articulating.
He has repeatedly embarrassed his superiors, saying, for instance, that putting Kenneth Clarke, the former Home Secretary, in charge of the police was like putting King Herod in charge of Mothercare, describing Barbara Mills, head of the Crown Prosecution Service, as a 'disaster', and suggesting her organisation be renamed the 'Criminal Protection Society'.
As police forces become more politically sensitive, his outspokenness has had increasing impact. Last week two CIB officers conceded his popular appeal. One said: 'He represents a lot of officers' views, but he doesn't represent all views and certainly not mine. He's too much like a bull in a china shop . . . the Terry Dicks of the Met.'
Mike Bennett, now 51, was brought up in a strict but happy Roman Catholic household in south London. His headmaster told interviewers for teacher- training college that he would have no problems getting the minimum five 'O' levels, but he didn't get them, and instead began work in a firm of solicitors - 'I left on principle after I found the law worked only for those who could afford it.'
He joined the police in 1961 and was promoted to sergeant after eight years. He was elected Met Federation chairman in 1986, thinking it would mean six meetings and one speech a year. Instead (although he still earns a sergeant's pay from the Met), it took over his life and he abandoned plans to take a law degree. 'I've been to more murdered police officers' funerals than anyone else in 'the job'. I've seen an awful lot of pain and suffering, but nobody seems interested.'
He could have retired on full pension 18 months ago. His wife would have him quit tomorrow. 'I don't know why I put myself up to be knocked down. Nothing changes.' But he is adamant about doing what he thinks is right. 'This investigation against me will show how seriously we take complaints. Whether it is a disciplinary matter is another thing.'
He has fought fiercely for policemen facing disciplinary panels. 'I used to play dirty. When you are pitting your wits against senior officers and expert witnesses and you get a result, it is satisfying.'
This anti-establishment stance has won him many admirers in the ranks. He has little time for Mr Condon's attempts to improve police relations with women, homosexuals and members of ethnic minorities at the expense of what he sees as the more fundamental issues of protection and support for officers: 'There are now two PCs in the Met - Paul Condon and political correctness.'
This prompted one Scotland Yard source, who did not wish to be named, to complain that he is a male chauvinist: 'He's great, if you happen to be a white, straight man.'
Some of his supporters, none of whom wished to be named either, questioned the motives behind the CIB investigation. 'You have to ask whether they are trying to do his legs,' said one colleague. Another said: 'It's obviously down to a certain person at the top. Bennett is a loose cannon, sure, but he speaks up for us and they want that to stop.'
Scotland Yard dismissed suggestions that the investigation was intended to gag Sgt Bennett. A spokesman said the decision would not have been referred to the Commissioner. 'The relationship between the Federation, superintendents and chief officers has been one of co-operation to extremely good effect. The Commissioner's relationship with the Federation is extremely positive and mutually beneficial.'
Last week, true to character, Sgt Bennett called in health inspectors over the prosaic matter of a mouldy canteen sandwich. 'My name is mud in this station now. Someone said I shouldn't have gone outside, they could have squared it. But there's too much of that sort of thing going on in this job.'