Police chiefs are to seek guarantees from politicians of all parties that they will be allowed to do their jobs free from “unwarranted political pressure”.

Ken Jones, president of the Association of Chief Police Officers, said that senior officers were extremely concerned that Sir Ian Blair’s sudden departure from Scotland Yard had “fundamentally altered the perception of policing independence”.

Mr Jones told The Times: “We see dangers in allowing a drift away from the fine balance of interests between government, chief officers and police authorities. In our country the duty to preserve the impartiality of policing rests squarely with us all. Politics, policing and narrow vested interests make for a toxic mix.” He added that police officers, like the judiciary, had a duty “to keep the Queen’s peace, free from undue influence”.

Mr Jones’s concerns were echoed by Sir Norman Bettison, Chief Constable of West Yorkshire, who ruled himself out of the running to succeed

Sir Ian. “The dislodging of Ian Blair is a demonstration of political will,” he said. “Along this road lies danger. I am therefore staying put.”

Another senior police chief said: “There is a line that the politicians must not cross — they must not start telling chief constables how to do their job. Operational independence is critical. If they cross that line, chiefs will walk away from the job.”

Sir Ian announced his resignation on Thursday one day after being asked to step down by Boris Johnson, the Conservative Mayor of London, who moved to oust him within hours of taking control of the Metropolitan Police Authority.

Mr Johnson’s actions have been criticised by Jacqui Smith, the Home Secretary, for politicising the top policing job in the country. The recriminations threaten to mire the process of selecting a successor.

Last night details of a letter from Mr Johnson to the Home Secretary emerged in which he wrote: “I would, however, counsel caution in moving too quickly to recommending a prospective post holder to Her Majesty.”

After noting that the Metropolitan Police had been passing through a period of “serious and potentially destabilising disruption at the most senior level”, he continued: “I feel that those of us with responsibility for ensuring the delivery of efficient and effective policing in London ought for the good of the organisation pause to take stock of the requirements of the post and to consider whether a fairly lengthy consolidation period, under the acting command of Sir Paul Stephenson, might not be for the best.”

Mr Johnson, who chairs his first meeting of the Metropolitan Police Authority on Monday, has a formal role in the appointments process. By law, however, the final appointment of the commissioner is made by the Home Secretary in a recommendation to the Queen.

With the appointment falling close to a general election, Ms Smith may also feel that she has to consult Dominic Grieve, the Shadow Home Secretary. That opens up the possibility of divisions within the Tories.

Speaking at the Conservative conference this week, David Cameron signalled that he was uncomfortable with Mr Johnson’s idea of stripping the Met of its national responsibility for terrorism in order to give the Mayor greater control over the force.

Mr Cameron asked: “Is change, is that big upheaval, the right thing to do when we are facing a terrorist threat?”

Mr Johnson denied having been motivated by party politics, saying it was a good opportunity for change. He said: “The decision that we came to — and I stress that it was his [Sir Ian's] decision following a conversation with me — the decision that he came to very much reflected the flavour of conversation that I had with a great number of people with responsibility for London policing and it was my view that I could not shirk the advice I was being given.”

The contenders and how the bookies rate them

Sir Paul Stephenson, 54, Deputy Commissioner, former Chief Constable of Lancashire. Already running day-to-day policing in London. Has made knife crime one of Met’s priorities. Odds 8-11

Sir Hugh Orde, 50, Chief Constable of Police Service of Northern Ireland. Straight-talking ex-Met officer who is credited with modernising policing in Ulster. 5-1

Mike Fuller, 49, Chief Constable of Kent, right. Had 25 years’ experience at the Met before becoming the first black officer to assume command of a force.8-1

Bernard Hogan-Howe, 50, Chief Constable of Merseyside. Another policeman with a no-nonsense approach. Waged successful campaigns against organised crime in Liverpool. 8-1

Sir Paul Scott-Lee, 55, Chief Constable of West Midlands. Highly respected in Police Service but thought to be interested in presidency of the Association of Chief Police Officers (Acpo) rather than top job at the Met. 10-1

Peter Neyroud, 49, chief executive of National Policing Improvement Agency. May be considered too similar in style to Sir Ian at a time when the Met wants “a copper’s copper”. 12-1

Ken Jones, 56, president of Acpo. Ran for the Met job against Sir Ian in 2004 and is expected to put his name forward again. 16-1

Betting odds: Paddy Power

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