Police are stepping in to take patients to hospital instead of ambulance crews at a time when resources are "stretched to near capacity", a national officers' group said.
Chairman of the Police Federation of England and Wales Steve Williams said officers are increasingly having to cover when an ambulance is required after an accident or crime.
He said: "We are noticing that the need for police to respond to emergency ambulance calls is no longer an isolated one. This is concerning in a time where resources for the police are stretched to near capacity.
"Let me be clear - the police will continue to respond to those who need us as soon as possible, and if this means responding when the ambulance service is unable to do so, then we will support our partners and the public eagerly.
"However, there remains a wider point here for Government and the emergency services in terms of the risk in not addressing the resourcing issue and wider public safety concerns. We would welcome Government's view as to how they will help us to address the problem."
The police group leader spoke after concerns were raised about ambulance services in the east of England.
Chairman of Essex Police Federation Mark Smith said his officers have to take patients to hospital nearly every day. He said: "We are having to convey to hospital almost daily in Essex at the moment. We are not pointing the finger at paramedics and ambulance crews, they are going through cuts as the police are going through cuts. The paramedics and ambulance staff join to do a job and I feel they, like us, are not able to do the job in the best way they can to serve the public."
One member told him about a case where a pregnant woman who had been assaulted was told she would have to wait four hours for an ambulance. She was taken to hospital in a police car which had to keep stopping so she could vomit because she was in pain.
Mr Smith said: "We are trained to give basic life support, we are not paramedics. We can hold the fort until trained professionals turn up. If they're not turning up and we've got seriously injured people we're not going to let those people down. But police officers have their own job to do and while we are conveying people to hospital we cannot do that job, and if the vehicle is covered in blood then it will be off the road. I want to raise this now and tell people what's happening so it can be tackled before people have to die."
A spokesman for the East of England Ambulance Service said: "While we have already revealed plans to improve our response times with a raft of measures, it is important to remember that patients get a response target according to thoroughly assessed clinical need with only around a fifth warranting the eight-minute target for life-threatening emergencies, the remainder being subject to a target of between 20 minutes to an hour."