Health Secretary Patricia Hewitt has defended plans to change emergency medical care, saying the new system would not put patients' lives at risk.
Patricia Hewitt says many people do not need to go to hospital
Under the plans, which follow a review of ambulance services, paramedics would treat some people at the scene instead of taking them to hospital.
She said those deciding whether to send paramedics or traditional ambulances should "err on the side of caution".
But the Lib Dems said giving people the best treatment had to be the priority.
Under the plan, ambulances would be reserved for emergencies, with cars or motorcycles sent to less serious cases.
Ms Hewitt told BBC Radio 4's The World This Weekend: "One of the things ... ambulance staff themselves and the management have to look at is that when that initial decision is made - do you send an ambulance or do you send an emergency care practitioner to treat at home - you err on the side of caution.
"If there's any risk at all that the patient needs to be sent into hospital then you send an ambulance."
Liberal Democrat health spokesman Steve Webb said the government should pilot the scheme before bringing it in.
"I would need to be convinced that this is about giving people the best treatment - not merely the cheapest treatment," he added.
And shadow health secretary Andrew Lansley said: "If we had a more integrated emergency service, this would help ensure that the most appropriate person deals with each case."
Ray Carrick from the Ambulance Services Unions said there were "health and safety difficulties" with the plan.
"If we've got staff going to an increased number of calls on their own, perhaps particularly female staff, then there are inherent difficulties attached to that and we need to be very cautious about how we approach it," he said.
But British Medical Association chairman James Johnson said the London Ambulance Service had already been using experienced emergency care practitioners to treat patients in their homes and that many people who called 999 did not need to go into hospital.
"Treating them at home not only saves money but also saves inconvenience," he said.
"No patient wants to go into hospital unless they really need to go into hospital."
Ms Hewitt told the Sunday Telegraph as many as 90% of calls to the emergency service could be dealt with in ways other than by rushing the patient to hospital.
She told the paper: "If you are a mum and your child has fallen off his bike, you want him checked over but the last thing you want is to take him to A&E.
"The ambulance service can take on a new role by taking the hospital to you."
Speaking to Sky News she said the measure was not a cost-cutting move but was instead a "real improvement".
She said staff would receive up to two years of extra training to qualify for the new role.
"This is about reshaping the ambulance service... about using the staff we have got better, training those who want to get more skills up so that they can actually do more for patients."
The review, by national ambulance adviser Peter Bradley, will be published on Thursday and will outline a five-year plan for developing the service.
It will say that up to one million patients a year are taken unnecessarily to accident and emergency departments.