Fire chiefs with no experience of fighting crime could head police forces in a drive to increase collaboration between emergency services.
The government will confirm on Tuesday that it is pressing ahead with measures to allow police and crime commissioners (PCCs) to take responsibility for their local fire and rescue authority.
PCCs who take this step will then be able to put in place a single "employer" led by a chief officer in charge of hiring all fire and police personnel.
The post will be reserved for those who hold the rank of chief constable but rules will be changed to allow senior fire officers who have not previously served as a constable to apply.
It is part of a wider strategy to foster closer working relationships between fire, police and ambulance services, although the government insists they will remain operationally independent.
Arrangements could include sharing headquarters and back office functions.
After a consultation, ministers have decided to introduce legislation that will place a statutory duty on all three blue lights services to collaborate.
The measures, which will only apply to England, will enable PCCs to take on the functions and duties of fire and rescue authorities in their area and potentially create a single employer for police and fire personnel "where a local case is made".
In the model, a chief officer would be accountable to the PCC for both fire and policing.
Beneath them a senior fire officer would lead fire operations, while a deputy chief constable would be appointed to lead police activity.
Officials stressed that fire officers will have to complete assessments and meet standards set by the College of Policing before they are eligible to become chief officers.
A law that prevents a member of a police force from being a firefighter will remain in force, while fire personnel will not be given the power of arrest.
A Fire Brigades Union spokesman said: "It is still our professional opinion that the plans to have PCCs take over responsibility for fire and rescues services are a mistake.
"PCC takeover of fire and rescue is a costly experiment with no guarantee for success."
Steve White, chairman of the Police Federation of England and Wales, also criticised the move.
He said: "Officers from both emergency services already do pull together, working alongside week in, week out, as has been evidenced most recently by the appalling floods.
"So why the burning need to change the law? It's like using a sledgehammer to crack a nut."
Policing and fire minister Mike Penning, said: "We believe that better joint working can strengthen the emergency services, deliver significant savings and produce benefits for the public.
"Strong leadership will be required to drive greater efficiencies and improved outcomes. Directly elected Police and Crime Commissioners are clearly accountable to the public and have a strong incentive to pursue ambitious reform and deliver value for money."
He added: "This is about smarter working. It simply doesn't make sense for emergency services to have different premises, different back offices and different IT systems when their work is so closely related and they often share the same boundaries."
Shadow fire minister Lyn Brown said fire officers will fear the plans are "a smokescreen for cuts".
She said: "The Government must ensure that a statutory fire service is maintained in all parts of the country."
College of Policing CEO Chief Constable Alex Marshall said: "Collaborations between the emergency services have already proven to benefit the public and we will support initiatives to make this process easier."
He added: "One of the proposals is around a chief fire officer applying for a role as a chief constable in single employer model force areas.
"We would be interested in seeing more from the Home Office on what would be required for a fire officer to reach the standard and professional qualifications in policing necessary to become a chief officer.
"Currently all chief police officers have to pass a rigorous selection process and training course in order to take up their post, and it is important this is not diluted."