Over coffee and croissants in a Westminster hotel in April, Boris Johnson, then the Tory candidate for London mayor, announced that the Metropolitan Police needed “a yank on the steering wheel and that’s what I intend to provide”.

Since then the Met vehicle appears to have careered off the road and crashed into the ditch.

Mr Johnson, now Mayor of London and chairman of the city’s police authority, has played his part in that crash. He dismissed Sir Ian Blair as Commissioner in an overtly political move that, he claimed, was aimed at ending the politicisation of the force.

Then he waded into the argument over the rights and wrongs of the investigation into the Tory immigration spokesman Damian Green – revealing that he had spoken to the suspect and was confident that nothing would come of it all. It was an extraordinary intervention for the chairman of a police authority to make in the middle of a hugely sensitive investigation. But Mr Johnson’s antics are nothing compared with the calamities that the Met’s top brass visit upon one another.

With its 50,000 officers and staff and a budget of £3.5 billion, Scotland Yard is viewed increasingly in Whitehall and Westminster as one of the most dysfunctional bodies in the public sector. That certainly matters in London, where the teenage murder rate has risen to a record level – but it also reverberates nationally.

The Met has nationwide responsibilities for counter-terrorism and is charged with the protection of the Royal Family and foreign dignitaries. Its commissioner is viewed as the leading voice for the police service across the country. The Met’s annus horribi-lis began in the shadow of a trial at the Old Bailey in which the force was found guilty of health and safety breaches in the operation that led to the death of Jean Charles de Menezes at Stockwell Tube station in 2005.

It has ended with another apology outside the Old Bailey, for the lapses that left the serial sex attacker Robert Napper free to kill three times, and with an inquest jury lambasting the force again over the death of Mr de Menezes. A year ago Sir Ian delivered the statement of apology outside court but announced defiantly that he had a job to do and that he was going to get on with it. He is no longer in that job.

Early this year Sir Ian faced allegations over his links with Andy Miller, an old friend and business consultant, who had been awarded Scotland Yard contracts worth £3 million. He robustly denied any wrongdoing but the Metropolitan Police Authority asked the Home Office to set up an independent inquiry. That is still continuing and has led to the indignity of a serving commissioner having his office searched and being questioned by detectives from an outside force.

Assistant Commissioner Tarique Ghaffur was at the centre of the next scandal to unsettle Scotland Yard. The country’s most senior Asian officer and a member of the senior management of the force made allegations of racism against it and Sir Ian personally. They were later dropped, in return for a six-figure settlement.

The willingness of Mr Ghaffur, who resigned from the Met, to don full uniform and make public allegations against his colleagues reflected a culture of infighting and indiscipline at the top of the organisation.

On the day that Mr Johnson took over as chairman of the police authority at the beginning of October, Sir Ian was called to a routine meeting at City Hall. The mayor told him that he did not have his confidence and within 24 hours Sir Ian was gone. The end of the Blair years was supposed to usher in a new era. Senior officers spoke of a unified management team and the restoration of a corporate responsibility.

The calm before the next storm lasted about seven weeks. Then officers from Assistant Commissioner Bob Quick’s Special Operations Command arrested the Conservative Party’s frontbench immigration spokesman.

Mr Quick had consulted Sir Paul Stephenson, the Acting Commissioner, before carrying out the arrest. Their conversation is said to have degenerated into a furious row, with Sir Paul questioning the need for an MP to be arrested and Mr Quick insisting that it was essential.

Bizarrely, neither man consulted the officers who had conducted the controversial cash-for-peerages inquiry (which drew intense political fire from Labour sources) and the Met seemed to be unaware that the legal basis of the leaking offences it had in mind were being disparaged in an important court case involving a journalist and a policeman that same week.

From that point until now, senior officers have been waging war with one another about the Green case. An external review paved the way by revealing “concerns as to whether elements of the investigative approach meet current policy and best practice”.

But Mr Quick is clinging to his right to persist with an investigation that he maintains is well founded. “The problem with the critics is that they don’t know what I know,” he told friends.

The pressure on Mr Quick seemed to tell at the weekend when he answered the phone to a reporter and delivered a salvo at the Conservative Party. “The Tory machinery and their press friends are mobilised against this investigation in a wholly corrupt way, and I feel very disappointed in the country I am living in,” he said.

Mr Quick has retracted his remarks but his inquiry remains live and, to his senior colleagues’ dismay, he is still reluctant to drop it. Sir Paul knows that it continues to undermine not just the Met but his chances of succeeding permanently to the position of commissioner.

An annus horribilis

— Investigation into award of £3 million contracts to a personal friend and skiing companion of Sir Ian Blair

— Allegations made by two senior Asian policemen of race discrimination and a “golden circle” of white officers

— Commissioner forced to resign from office for the first time in 120 years, after a disagreement with the Mayor of London

— Sir Ian leaves the Met with a reported £300,000 payout, amid claims he used the cash-for-honours affair to gain leverage over the Home Office

— Murders of teenagers reach record levels amid an epidemic of gang and knife violence

— Stockwell inquest jury rejects police officers’ account of the shooting of Jean Charles de Menezes in 2005 after the failed 21/7 attacks in London

— Public apology to Colin Stagg for charging him with the murder of Rachel Nickell and leaving Robert Napper free to kill again

— Arrest of Damian Green and the sending of 20 antiterror officers to search the MP’s offices in the Houses of Parliament, sparking a debate over policing methods

— The Assistant Commissioner, Bob Quick, apologises after claiming that the “Tory machinery and their press friends are mobilised against this [Green] investigation in a wholly corrupt way”.

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