Police cars with no sirens are being used for emergency responses, delaying officers and potentially preventing arrests, the Police Federation says.
West Midlands Police has a fleet of 109 Vauxhall Corsa cars intended for neighbourhood policing.
The federation, which represents rank and file officers, said the vehicles had been used for more than 100 urgent calls in the past 12 months.
The West Midlands force said the Corsas were “not intended for response work”.
The federation's health and safety official Pete Harkness said: “Officers are ordered to drive the Corsa at patrol speed and obey the rules of the road.
“But the absence of a siren means they often cannot get traffic to move out of their way and this, in turn, can delay them getting to an emergency situation.
“This is very frustrating for the officers and they feel they are letting the public down by not getting to them as quickly as possible in a crisis situation.”
The federation blames cost-cutting for the use of cars without sirens.
Supt Kerry Blakeman, from the force's operations department, said officers who respond to call-outs in cars without sirens have to follow the Highway Code and cannot break the speed limit to cut through traffic.
He said: “Incidents graded as 'immediate' are responded to as soon as possible, usually in a matter of minutes and on many occasions require the use of lights and sirens.
“At no point was the Corsa intended for response work as it is a low performance vehicle.”