Conservative backbencher Mark Reckless has called on ministers to ensure that elected police commissioners will not be able to influence "individual investigations and arrests".
The government plans to hold elections for residents to choose their local police commissioners in England and Wales from May next year.
But as MPs debated the Police Reform and Social Responsibility Bill at report stage on 30 March 2011, Mr Reckless urged MPs to back his amendment which would enable police forces to operate independently of any interference from the new commissioners.
Policing Minister Nick Herbert argued that the amendment was unnecessary, since the "direction and control" of police forces would remain with chief constables under the bill.
He conceded that there was "concern" about operational independence, but warned that it would be a mistake to try to define the principle in law.
Under the plans, commissioners will have the power to hire and fire chief constables, publish a police and crime plan and set local police and crime objectives.