A great city is a magnet for crime as well as enterprise. For its people to feel safe it needs a police chief who can inspire those in his command and whose authority is unquestioned. Sir Ian Blair understands this. “Every member of the Met works to my direction and must meet my reasonable requirements,” he insisted last month. His requirements may be reasonable, but his direction has failed to inspire.

This conclusion has little to do with allegations of racism that will force Sir Ian to appear today at an employment tribunal in London. The case inevitably recalls the Macpherson report nine years ago into the Metropolitan Police Service's handling of the Stephen Lawrence murder investigation. The report branded the force, and British policing generally, “institutionally racist”. Today Sir Ian is accused by Shabir Hussain, a senior Asian officer, of racism in not promoting him above the rank of commander. A second tribunal involving similar claims by Tarique Ghaffur, Britain's most-senior Muslim policeman, could follow.

Some will infer that little has changed in the Met's attitude towards racism since the Macpherson report. The facts of these cases will emerge in due course. Elsewhere, though, the signs are positive. The proportion of newly recruited Metropolitan police officers drawn from ethnic minorities has risen from 5 to 21 per cent in five years. Just as significantly, fewer ethnic minority than white recruits now drop out during training.

These trends have continued on Sir Ian's watch and he deserves some credit for them. His problems in defending his position relate less to race than simmering dissatisfaction among his senior staff.

Since taking up his post in 2005 Sir Ian has lost several of his most capable assistant commissioners, among them Andy Hayman, in charge of Special Operations, and Peter Clarke, a highly respected head of Counter-Terrorism Command, who was the Met's public face after the 7/7 bombings. The Commissioner could illafford to lose both men, as he did, in quick succession.

More recently, muttered disloyalty towards Sir Ian among senior officers has verged on open dissent. This is no way to run the country's biggest police force, especially when the public's overwhelming concern is not competing egos at Scotland Yard, but soaring knife crime.

Last year 26 teenagers were murdered in London - a figure Sir Ian rightly called unacceptable. Nineteen more youngsters have been fatally stabbed or shot in the first half of this year alone. In these circumstances it is little consolation that knife crime is now a top police priority; nor that most categories of crime, including murder, rape and robbery, are falling. This is partly because, as Sir Ian admits, the public has long since stopped trusting most crime statistics. But it is mainly because as long as teenagers are dying on the streets, parents fear viscerally for their own and their children's safety.

No one disputes that the organisational changes Sir Ian has launched, including abolishing the Special Branch, have caused some of the criticism from within his ranks. But his tenure has been defined by crisis management, not strong and steady leadership. In the process he has failed to win the loyalty of enough of his most senior officers, or the unqualified support of the new Mayor of London.

This is the context for his next challenge, the inquest into the unconscionable shooting death of Jean Charles de Menezes. It will prompt fresh calls for Sir Ian to resign. The most worrying fact for the commissioner, though, is that increasingly few men and women in his force would urge him to resist them.