Vauxhall has revealed new evidence from its investigation of fires involving the Zafira model, as it upgrades its fix to full safety recall status.
The car manufacturer has told ITV News its tests have shown worrying levels of "improper repairs".
Vauxhall has now tested almost 1000 components selected at random from Zafira B cars and found 26 were "manipulated" making them dangerous.
The parts, checked at the manufacturers' German labs, were thermal resistors - a type of fuse designed to protect the heater system.
The company's new evidence suggests unauthorised repairers have used a variety of techniques to disable the resistors - leaving cars vulnerable to fire.
The firm says the work was "undertaken outside of Vauxhall’s authorised service and maintenance processes and schedule".
Officials have now asked Vauxhall to start a full safety recall.
This is a formal reclassification that will have little practical effect on the company's existing actions to inspect and repair all 234,938 Vauxhall Zafira Bvehicles in Britain.
Owners are being asked to take their cars to a retailer for technicians to inspect the heating system and replace resistors in all the vehicle.
They company is doing this, free of charge, because it says it is not always possible to spot dangerously repaired resistors.
Many members of a Zafira owners group on Facebook still believe the cars have a design fault and have vowed to continue their campaign.