During a joint operation by the police, South Gloucestershire Council officials and the Vehicle and Operator Services Agency (VOSA), 17 taxis were stopped.
Six had such serious faults – from illegal tyres and defective seat belts to lights that did not work – that prohibition notices were served on them.
A further four were on the borderline of legal requirements, while the remaining seven were satisfactory.
The operation was aimed at protecting taxi users over the busy Christmas season and included checks on tax, vehicle insurance, tyres, lights, seat belts, tinted windows and any other area which gave cause for concern.
Police motorcyclists were involved to identify licensed South Gloucestershire taxis and escort them to Kingswood Civic Centre, where they underwent mechanical checks by VOSA and additional checks with licensing officers.
Mark Pullin, trading standards and licensing manager, said: "We are determined to ensure that local taxi operators adhere to the highest standards in order to protect the public."
Heather Goddard, the council's executive member for communities, said: "Many South Gloucestershire residents will be using a taxi this festive season so it's essential that these vehicles comply with all legal requirements governing their use.
"Public safety is of paramount importance, so we will not hesitate to take action when taxi operators fail to maintain their vehicles properly."