All of the region's ambulances are to be deep-cleaned following the report
An inspection has revealed the North West Ambulance Service (NWAS) is putting patients at risk of infection with dirty vehicles and poor training.
The Care Quality Commission (CQC) looked at 22 ambulances and found nine were "in a very poor condition with visible dirt".
The other 13 were not cleaned to a good standard and one was so unclean it was taken out of service, the CQC said.
The ambulance service said it took the report "extremely seriously".
The NWAS covers the Cumbria, Lancashire, Manchester, Merseyside and Cheshire areas.
Inspectors found most staff were unaware of the ambulance service policy on cleanliness.
Stations were visited in Bolton, Bury, Stockport, Newton-le-Willows, Whiston, Warrington, Chorley and Darwen.
The hospitals inspectors also looked at ambulances at hospitals including Fairfield in Bury, Manchester Royal Infirmary, Whiston, the Royal Liverpool and Lancaster Royal.
The report said: "Nine out of the 22 (41%) were found to be in a very poor condition with visible dirt on the floors, walls, horizontal surfaces and door frames.
"The remaining 13 were not found to have been cleaned to a good standard; all had some areas of dirt in the interior of the ambulance.
"It was clear from the level of dirt these vehicles were not cleaned on a frequent basis. All crew reported that none of the vehicles we inspected had had a deep clean in the past year or more."
Professor Kevin Mackway Jones said the admitted the report was "damning"
One ambulance at Stockport had what appeared to be blood stains on the walls and had "ingrained dirt".
This vehicle was removed from service by the CQC during the inspection to be deep cleaned.
The report found the ambulance service trust had a clear policy on cleanliness and managers to lead it. It also found that antibacterial washes were adequate and both staff uniforms and laundry reached the required standards to prevent infection.
But, six out of 10 grab bags, which are used to store equipment, were dirty with torn or damaged exteriors, the report said.
The NWAS has now brought forward a programme to replace of them.
It also plans to deep clean all of its ambulances by mid-September and is setting out a clear programme of audits for its vehicles.
Professor Kevin Mackway Jones, medical director at NWAS said: "We are taking this inspection report extremely seriously.
"The safety of both our patients and staff is an absolute priority to us and we have already put a number of measures in place to ensure that our vehicles meet the highest standards of cleanliness."
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