Ambulances may be spreading infections because they are not being cleaned properly, union leaders warn.
Guidelines for ambulance cleanliness are voluntary
An investigation by Unison found large variations in cleaning practices at ambulance trusts in the UK.
Some areas do not have dedicated cleaning staff and ambulance crews are left to give the vehicles a "quick mop out", the health union said.
The government said it expected trusts to follow national guidance on ambulance cleaning.
Unison called for cleanliness standards to be properly applied and monitored across trusts to improve standards.
Ambulance crews questioned by the union said targets, time and money were all to blame for different cleaning practices between trusts. In the worst example, ambulance crews in the North West have no dedicated cleaning staff or cleaning time.
Paramedics report that they do not get time to even check the vehicles, which are never deep cleaned.
The only precaution is that if staff know a patient is infected, for example with Clostridium difficile, the ambulance will be mopped out.
At the other end of the scale London Ambulance Service has introduced on-site cleaners who work throughout the night to routinely deep clean the fleet.
They also restock ambulances with fresh kit, freeing up paramedics for seeing patients. Effectively, crews bring in a dirty vehicle and leave with a cleaned one, Unison said.
In the South West, funding problems are causing delays to plans to employ staff to carry out deep cleaning, staff said.
And paramedics in Wales agreed not enough was being done although the service is looking at a similar deep clean system to that used in London. Sam Oestreicher, Unison national officer for ambulance staff said: "The government recently announced extra money for deep cleaning hospitals, but ambulances seem to have been forgotten.
"They are part of the patient care package and no one should have to travel or work in a dirty ambulance.
He added: "In many trusts, ambulance crews are responsible for cleaning their own vehicles and this is a waste of their time and training.
"They should be out there saving lives not mopping out the back of a dirty ambulance."
He said existing voluntary guidelines should become mandatory standards.
But stressed that extra funding would be needed if or resources would be cut elsewhere.
Health Minister Ben Bradshaw said: "We take healthcare associated infections and cleanliness seriously in all healthcare environments.
"The available evidence does not suggest that ambulances are a major source of infection, but tackling healthcare associated infections is an NHS-wide issue.
"The Ambulance Service Association has issued guidance and we expect trusts to follow this. "Trusts should also take cleaning time into account when managing their supply of ambulances to attend emergency calls."