Hampshire police chief Simon Hayes has backed a campaign against plans to allow minicab owners to lend their vehicles out when not at work.
Police Commissioner Hayes is one of 19 commissioners across the country who say that rape and sex assaults could rise under the new proposals.
The cross-party group of Police and Crime Commissioners (PCCs) across England and Wales said planned deregulation will lead to a “free-for-all”.
Currently, only licensed private hire drivers can drive a marked private hire vehicle. Drivers have to be regularly re-licensed and there are restrictions on vehicles operating across local authority boundaries.
Under the Deregulation Bill, the rules are to be swept away, with anyone allowed to drive a private hire vehicle for their own use when the registered driver is not using the car.
The Government says this will free many families from the need to run a second car and save them money.
But critics say it means there will be no guarantees that the driver of a vehicle is who they say they are.
Now the commissioners, led by Greater Manchester Police and Crime Commissioner Tony Lloyd, have joined forces to lobby Ken Clarke, the minister responsible for the Bill, to stop the changes.
Mr Lloyd said: “The current system is far from perfect, but at least there are safeguards there which mean people can have confidence that, when they get into a marked private hire vehicle, it is genuine and being driven by a licensed operator.
“Taking these safeguards away opens the prospect of a private hire free-for-all.
“There will be no guarantee that a driver is who he says he is, and the inevitable consequence is that there will be an increase in people being attacked after a night out.”
A spokesman for the Department of Transport said: “The Deregulation Bill will not put taxi passengers at risk and drivers will continue to have their backgrounds routinely checked.
“Councils will have strong tools to assess drivers’ and operators’ suitability and to carry out enforcement activity.
“The Disclosure and Barring Service will allow licensing authorities to discover any new convictions during the lifetime of a driver’s licence.”