Thames Valley Police could be left without a Chief Constable for up to two years while plans to merge the force with Hampshire are explored.
Peter Neyroud is to vacate the top job in January when he becomes the first chief executive of the new National Policing Improvement Agency.
But the force has been barred from appointing a successor pending the outcome of restructuring proposals, which will not be complete until 2008.
Thames Valley Police Authority is now in talks with Sir Ronnie Flanagan, chairman of the Senior Appointments Panel, about interim arrangements.
A dogfight is already expected between Britain's top police officers, who face losing their jobs when the number of forces is cut from 43 to as few as 12.
The shake-up of police forces across the country was announced in September after Her Majesty's Inspectorate of Constabulary published a report which found that the current structure was no longer "fit for purpose".
Thames Valley Police have been asked to submit proposals to the Home Office by December 23.
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