Scotland Yard is recognised throughout the country. As the shorthand given to the Metropolitan Police Service, the Yard is often said to be “called in” when other police forces are in need of support. For some purposes, the Met really is a national police force, having responsibilities on counter-terrorism and other serious crime for the whole of the country.

Sir Ian Blair's resignation is, therefore, of importance for people living in Leeds and Birmingham as well as in the capital. Operations are set up and managed from SW1 that affect the lives of, say, groups linked to terrorist activity in Yorkshire or the West Midlands. The size of London and the force's national role inevitably give power to the Metropolitan Police that exceeds that of any other constabulary. Small wonder that former Met officers find themselves as chief constables in places as far-flung as Strathclyde, Northern Ireland, Merseyside and Kent.

The long chain of events that has led Sir Ian to quit includes the killing of Jean Charles de Menezes, questions raised over contracts given to a friend of the commissioner and, most recently the disastrous race-tinged, spat with Assistant Commissioner Tarique Ghaffur. Sir Ian had allowed himself to be seen as a “new Labour” police chief, with a liberal, intellectual, approach to policing. Many of his enemies believed he was too close to the present Government's concerns about, say, equality. The fact that he leaves the Met struggling with a number of high-profile race discrimination cases is paradoxical indeed.

The Conservatives have been calling for his resignation for more than a year. Both the Tories and the Liberal Democrats on the Metropolitan Police Authority wanted him gone. It is without precedent for Opposition parties to argue for the dismissal of the country's chief police officer. The three-year period of his tenure has been bad for the Met's reputation. Most recently there have been competitive leaks to the media by camps surrounding the commissioner and Mr Ghaffur. The fall-out from the de Menezes inquiry will make things worse, as senior officers are likely to end up giving evidence against each other.

A war of all against all is not good for policing. Morale in the Met must have suffered. In fairness, recorded crime figures in London have declined in recent years, though even this achievement is clouded by the lack of public belief in the statistics. Worse, perceptions of criminality have not fallen in line with recorded crime numbers.

The dust will take a long time to settle on this momentous affair. Boris Johnson has used his authority as Mayor of London to depose the Government's appointed London police commissioner. Such a move tells us much about the legitimacy acquired by the mayor's vast mandate. Equally, that Sir Ian evidently felt that Jacqui Smith, the Home Secretary, would not or could not defend him suggests that the Government is now weak in relation to the mayor.

Whitehall will have to find a new commissioner (formally the appointment lies with the Queen). There will be no shortage of candidates, though Sir Ian's successor will have a huge job. For a start, it will be necessary to re-establish the political independence of the office. This is not to say that Sir Ian was personally political, but that his handling of the office led to a situation in which Labour supported the commissioner and the Tories opposed him. Police chiefs must command the support of all political parties. The new appointee will also have to improve morale in the ranks. Many junior officers feel very distant from the elite at the Yard.

Such a momentous change at the top will also provoke a renewed debate about whether or not it is appropriate for the Met to combine the duties of a London police force with national responsibilities of various kinds. From time to time there have been proposals for a British FBI. Latterly, national quasi-police agencies have been created, such as the Serious Organised Crime Agency and, for immigration, the UK Border Agency. The railways are policed by the British Transport Police, a national squad.

Turmoil at the Met is bound to reignite a debate about whether the country, and the capital, would be better served by separating London's local policing from an array of national functions.

The Met will fight to defend its existing mix of responsibilities. It will argue that the capital is so important to the UK as a whole, and such an evident gateway for various kinds of serious crime, that it would be disastrous to create an FBI-type force, cut off from the London force. However, such a reform cannot be ruled out.

Similarly, there will be a discussion about how far politicians should appoint or control the operational activities the police. The Conservatives have put forward the idea of directly-elected “sheriffs” - police chiefs who would stand for office.

What happened in London yesterday shows that Boris Johnson, as mayor, effectively has hire-and-fire powers. Any new commissioner will have to have his seal of approval. Why not go the whole hog and allow the mayor to make the appointment?

Gordon Brown's Government has been looking for new policies to distinguish it from the resurgent Tories. There is a powerful case for looking at the way police accountability works in Britain. If politicians, with their electoral mandate, were more closely involved in appointing police chiefs and, more controversially, in determining operational priorities, surely it would strengthen the authority of an individual constable when confronted with wrongdoers. Not having someone with elected legitimacy standing behind a police officer on the beat weakens him. We are long past the cosy, cap-doffing days of George Dixon.

These are strange times. So powerful was the impact of Sir Ian's resignation that it pushed the collapse of the world financial system off the top slot in the online versions of virtually all the national newspapers. The world of economics and politics are simultaneously under extreme pressure. Sir Ian Blair's demise is another sign of the times.

Tony Travers is Director of LSE London

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